Responsiveness in an Era of Inequality

To what extent do members of Congress respond unequally to people in different economic situations? How does partisan control of the agenda change the way in which Senators respond to the poor? Using data from the 2004 National Annenberg Election Survey, and multiple roll call votes, I examine Senate responsiveness for the 107th through 111th Congresses. The results show consistent responsiveness toward upper income constituents. Moreover, Republicans are more responsive than Democrats to middle-income constituents in the 109th Congress, and a case study of the 107th Senate reveals that responsiveness toward the wealthy increases once Democrats take control of the chamber.


Regular Article
The growth in wealth and income inequality is one of the most pressing problems currently facing American society. Although wealth and income inequality has grown in the post-World War II era, the process has accelerated in recent decades. Since the 1980s, the growth in income and wealth among the top 5 percent has increased substantially, yet the other 95 percent have seen little or no increase (Piketty and Saez 2003). Researchers who have studied growing inequality have even labeled the recent era a "New Gilded Age" (Bartels 2008). With the growth in income and wealth inequality in America since the late 1960s, scholars, policymakers, and advocates for disadvantaged persons are increasingly concerned that inequality among citizens could be a mounting threat, as disparities in economic inequality often contribute to unequal political outcomes (Task Force on Inequality and American Democracy 2004). Although some research has been done investigating the causes and consequences of wealth inequality (e.g., Bartels 2008;Gilens 2005; see also Keister and Moller 2000 for a review), very little has been done to investigate how the most recent increase in inequality affects representation in the American political system. The fact that little has been done is surprising, given that the idea that all citizens are to be treated equally is central to most standard theories of democracy. Although liberty and popular sovereignty also constitute essential aspects of republican democracy, equality among citizens remains a chief concern for democratic theorists (e.g., Dahl 1956).
One way to evaluate how wealth inequality affects American democracy is to examine the degree to which the government responds equally (or unequally) to citizens of different economic situations. The concept of representation has become central to contemporary democratic theory. Although an essentially modern concept, representation has come to mean popular representation and is now linked with self-government (Pitkin 1967). In the representation literature, there is a considerable focus on responsiveness-the level of correspondence between constituency preferences and a legislator's behavior (e.g., Achen 1978;Miller and Stokes 1963).
This article contributes to recent work of scholars investigating the causes and consequences of unequal representation (e.g., Bartels 2008). Using data from the 2004 National Annenberg Election Survey (NAES), I examine the responsiveness of Senators to different economic groups across a range of issues for the 107th through 111th Congresses. 1 This time period allows me to examine responsiveness under a broad variety of institutional circumstances. These circumstances matter because they might affect the inequality that occurs through mechanisms like differences in party control of Congress, the 459567P RQXXX10.1177/1065912912 459567Political Research QuarterlyHayes 1 Trinity University, San Antonio, TX, USA nature of different agendas, as well as differences in behavior across chambers. In addition, I use the 107th Congress as the most similar (within) case study and examine how a change in partisan control of the agenda might change the way in which different groups receive representation.
In the analysis that follows, I expand on the work of scholars who have examined unequal governmental responsiveness (Bartels 2008;Gilens 2005). My analysis covers a broad period in which inequality has been especially pronounced and partisan control of the Senate has changed often. This allows for an examination of the way in which institutions are responsive to the poor. Researchers have learned that wealth inequality has increased substantially since the time of previous analyses, which is even more reason to examine the extent to which this inequality is affecting governmental responsiveness (Piketty and Saez 2003). This phenomenon can be seen in Figure 1. The figure shows the degree to which incomes are unequal for different quintiles of Americans. As the figure shows, those in the three lowest quintiles (the bottom 60%) have seen little change over time. However, most of the dramatic gains in income have gone to the highest quintile. Moreover, among the top quintile, incomes have increased dramatically for the top one and five percentiles. Bartels' analyses covered the Senate from 1989 to 1994 and are, in effect, over 20 years old. Although income distributions were unequal during that time period, the figure demonstrates how inequality was much greater during the period of my analyses (2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010). In fact, one of the largest increases in income transfers to the top 1 percent occurs after 2002. Therefore, the time period for which I examine Senate responsiveness is a period of extreme inequality in America, a period in which we might expect greater responsiveness toward those with the most resources.
My analysis also examines partisan control of the agenda that has been shown to affect the types of issues considered in Congress (Cox and McCubbins 2005). Although the Congresses studied by Bartels cover both a period of divided government (101st and 102nd) and of unified control (103rd), the Senate was controlled by the Democratic Party throughout. The period for which I examine Senate responsiveness (107th-111th) is during a time in which the Republican Party controlled the Senate and had unified control of the government for almost the entire period for the 107th, 108th, and 109th Congresses. By including the 110th and 111th Congresses, it is possible to examine a period for which the Democratic Party controlled the Senate. Moreover, the inclusion of the 111th Congress allows for a view of the Senate when the Democratic Party had unified control of the government, as the 2008 election produced large gains in both chambers of Congress as well as Barack Obama winning the presidency.
Overall, this time period of my analysis provides an interesting window to examine the degree to which changes in partisan control of the Senate might lead to different types of responsiveness to different economic groups. Because the Democratic Party has historically sought to represent disadvantaged groups and favor policies that incorporate leveling (while Republicans have largely opposed such measures), it is necessary to examine the extent to which partisan control can affect representation of different income groups, especially the extent to which partisan control of the legislature can affect agenda setting (e.g., Bartels 2008;Cox and McCubbins 2005). Alternatively, as the growth in wealth and income inequality has increased during times when both major parties have controlled the legislature, and given that both parties receive the majority of campaign contributions from the wealthy (e.g., Verba, Schlozman, and Brady 1995), partisan control of the legislature might not lead to differential responsiveness.
The main findings of this article are fourfold. In the analysis that follows, I find the following: (a) There is significant responsiveness toward upper income constituents in each Congress I examine; (b) regardless of which party controls the Senate, I am unable to detect responsiveness toward low-income constituents; (c) Republicans are more responsive than Democrats to middle-income constituents in the 109th Congress; and (d) in a case study of the 107th Senate, responsiveness toward the upper income constituents increases once Democrats take control of the chamber.

Legislator Responsiveness
The most common way in which representation has been studied is dyadically, examining the relationship between an individual member of Congress and his or her constituency. A significant number of studies find legislators to be responsive to median constituent preferences (e.g., Bianco, Spence, and Wilkerson 1996;Erikson 1978;Fenno 1978). However, numerous studies find contradictory results arguing legislators are not especially responsive (Bernstein 1989;Dennis 1998;Dennis, Medoff, and Gagnier 1998;Page et al. 1984). Still other studies find mixed results (e.g., Achen 1978;Elling 1982;Hutchings 1998;Miller and Stokes 1963).
A growing perspective examines legislator responsiveness to smaller groups within a constituency. This perspective argues that rather than simply responding to the majority preferred view of an entire district, members of Congress respond to smaller groups or subconstituencies (Bishin 2000(Bishin , 2009Clausen 1973;Clinton 2006). In many ways, this perspective closely aligns with Fenno's (1978) famous distinction between geographic, electoral, primary, and personal constituencies. This perspective offers a useful way to examine the extent of legislator responsiveness, especially if we want to know how well the government responds to groups equally (or unequally). Therefore, instead of examining responsiveness to an entire state constituency, this article divides a state constituency by economic class. I do this largely because it is an important focus for the study of representation to examine the extent to which legislators respond to various groups that comprise their constituencies in an equal manner, especially because this is one of the primary aims of democratic governance (e.g., Dahl 1971).

Biases in Representation
For decades, scholars have questioned the extent to which the government responds equally to citizen concerns. For example, Schumaker and Getter (1977) discover a bias toward spending preferences of upper class whites in a survey of cities. Hill and Leighley (1992) find a class bias toward spending preferences of upper class citizens in terms of state welfare spending. Martin Gilens (2005) investigates which group preferences are most influential in shaping policy decisions and finds that when high-and low-income groups disagree on policy, the poor are consistently ignored in favor of high-income Americans' preferences.
The most expansive study of unequal representation was conducted by Larry Bartels (2008), who tests the degree to which there are biases in Senate representation. Bartels finds that Senators, although quite responsive to middle-and high-income groups, are largely unresponsive to low-income groups. Bartels also finds partisan differences in representation, as Republicans are more responsive than Democrats to the views of high-income constituents. The author also finds that for either party, there is no evidence of responsiveness to the views of constituents in the bottom-income group. Bartels tested Senator responsiveness from 1989 to 1994, a period in which the Democrats retained majority control during each Congress and when wealth inequality was not as pronounced as in more recent times.
One of the most common explanations for why the poor are not well represented in American politics is that those in the lower classes often participate in politics at much lower rates than do middle-and upper income citizens (e.g., Verba, Nie, and Kim 1978;Verba, Schlozman, and Brady 1995;Wolfinger and Rosenstone 1980). Wealthier citizens generally have a higher propensity to vote, contact public officials, volunteer for campaigns, write letters, attend protests, and donate money to candidates (Verba, Schlozman, and Brady 1995). In addition, some argue that the wealthiest of the wealthy (e.g., top 0.01%) successfully push their tax burden on the less rich strata (e.g., Graetz and Shapiro 2005;Winters and Page 2009). Moreover, some have found that voters are better represented in the political system than nonvoters (e.g., Griffin and Newman 2005) and that federal spending is higher in areas where voter turnout is higher (e.g., Martin 2003). Similarly, studies show that there are gaps in the political knowledge and interest between rich and poor Americans (Converse 1990;Delli Carpini and Keeter 1996). The vast array of research examining the participatory differences therefore suggests that there are a number of reasons why wealthier citizens could be better represented than the poor. Despite these findings, there is little evidence that these differences contribute to responsiveness disparities. The little research on this subject that exists finds that differences between the rich and poor in voting, contact, and knowledge are uncorrelated with disproportionate responsiveness toward the wealthy (Bartels 2008).
Additional explanations exist that might suggest biases in representation. Research suggests that partisan differences in the control over the economy lead to vastly different outcomes for different groups (e.g., Bartels 2008;Hibbs 1977;Hibbs and Dennis 1988). Cleary, economic philosophies and priorities of Democratic and Republican administrations often diverge. This can often be seen in party platforms and different political ideologies set forth by the two parties (Tufte 1978). As the class composition of the major parties' supporting coalitions differs in the United States (as in many other nations), differences in macroeconomic policies often follow as well (e.g., Hibbs 1977). Moreover, Hibbs and Dennis (1988) present evidence that differences in distributional goals of the major parties affect economic distributions of Americans through policy-induced variations in macroeconomic policy. As each party has different goals when controlling the government, the partisan control of the agenda might be an important factor in determining who gets represented and who does not (e.g., Cox and McCubbins 2005).
In a related manner, although popular accounts of recent voting behavior suggest that the Republican Party has convinced the poor to vote contrary to their material self-interest by casting votes for conservative candidates based on social issues, empirical evidence in political science literature, however, calls this assertion into question (Bartels 2006(Bartels , 2008Gelman et al. 2008;Stonecash 2000). Bartels (2008), for example, finds no evidence that contemporary American politics is driven primarily by cultural issues, that working class whites have abandoned the Democratic Party or become more conservative, and that religious voters are distracted from economic issues. There is evidence, however, that voters behavior on pocket-book issues is often unsophisticated, ideologically confused, and myopic (Achen and Bartels 2004;Bartels 2008).
In sum, this literature review provides some reasons to believe the poor might not be represented as well as other groups. Although I am not able to test for all of these causes due to data limitations, this brief summary of the literature shows that there is good reason to believe that the rich would receive better representation than the poor. First, previous research demonstrates that the wealthy are more likely to receive better responsiveness than the poor (e.g., Bartels 2008;Gilens 2005). Second, previous research finds that Republicans are more likely to respond to the upper class than Democrats, thus demonstrating the likelihood that there are partisan differences in responsiveness across income groups (e.g., Bartels 2008;Hibbs 1977;Hibbs and Dennis 1988;Tufte 1978). Finally, there seems to be evidence that partisan control of the agenda might affect the degree to which different groups of voters receive benefits and/or responsiveness from government officials (e.g., Cox and McCubbins 2005;Hibbs and Dennis 1988). However, little is known about the degree to which unequal responsiveness occurs in the contemporary era. Therefore, in the following section, I detail hypotheses that come from these key expectations in the literature and then explain how I set out to test them.

Expectations
I test three main hypotheses in this article. First, I test whether Senators are more likely to respond to the preferences of upper income constituents. The Unequal Responsiveness Hypothesis predicts that, all things being equal, Senators will be more likely to respond to the preferences of upper income constituents. This expectation largely stems from previous findings (Bartels 2008;Gilens 2005), as well as from the participation literature, which generally finds the wealthy to have vastly higher rates of participation in politics (e.g., voting, volunteering, donating) than low-income citizens (Verba, Nie, and Kim 1978;Verba, Schlozman, and Brady 1995;Wolfinger and Rosenstone 1980). My second expectation is a test of partisan biases in representation. The Partisan Bias Hypothesis anticipates that Republicans will be more responsive than Democrats to the preferences of high-income groups. This hypothesis stems from research that finds differences in the coalitions and distributional goals of each party (Bartels 2008;Hibbs 1977;Hibbs and Dennis 1988;Tufte 1978). Therefore, due to the traditional manner in which the parties have represented different groups of voters, I expect Democrats to be more responsive to lower class individuals than Republicans.
My third expectation is a corollary of the Partisan Bias Hypothesis. This expectation focuses on partisan control of the Senate and therefore the legislative agenda (Cox and McCubbins 2005). Research in representation has often assumed that members of Congress look outward for cues (e.g., Kingdon 1977). Thus, agenda control is likely to matter for dyadic representation as Senators' responsiveness to their constituency (or constituencies) might be influenced by the issues that reach the floor. Therefore, the Agenda Control Hypothesis predicts that, all else equal, when Democrats have majority control in the Senate, those in the lower classes will receive better representation. In contrast, when Republicans control a majority of seats in the Senate, this hypothesis predicts a greater level of responsiveness to upper income constituencies. Specifically, I expect higher responsiveness toward the wealthy when the Senate is controlled by the Republicans (108th and 109th) and more responsiveness toward the lower class during sessions controlled by the Democrats (110th and 111th). 2 In the following section, I detail the data and methods I use to estimate Senator responsiveness.

Data and Method
The data I use have a number of advantages over other sources. To measure constituency opinion, I use the 2004 NAES. 3 For most issues, measures of constituency opinion needed to assess the influence of opinion on responsiveness across districts and states do not exist. When surveys do ask citizens for their preferences on specific legislation, there are seldom enough respondents to obtain accurate measures of constituency opinion. Luckily, the 2004 NAES helps overcome this problem with more than 90,000 respondents interviewed during the course of the election campaign. In addition to data from the 2004 NAES, I use NOMINATE data to estimate Senator ideological orientation and The Almanac of American Politics to identify Senator party membership. Detailed descriptions of each variable used in the analysis are presented in online Appendix A.
To gauge constituency opinion and evaluate the degree to which Senators respond to different income groups, I separated respondents in the Annenberg Survey into terciles: a low-income group with household income below $35,000, a middle-income group with income between $35,000 and $75,000, and a high-income group with income above $75,000. 4 These groups constitute 33, 37, and 29 percent of respondents, respectively. Next, I estimated the average opinion of survey respondents within each state (by income group). I estimated constituency opinion using a proxy measure, using respondent ideology on a traditional liberal/conservative scale as a measure of income group opinion. 5 This scale ranges from −2 to 2, with lower values coded as liberal and higher values coded as conservative.
Estimating opinion by income groups using survey data is somewhat problematic due to the possibility of measurement error. Because most surveys do not include enough respondents to accurately measure state or district opinion, this often creates a large degree of measurement error (see Achen 1978). However, for this analysis, the 2004 NAES interviews a large number of respondents in each state, ranging from a low of 184 in Wyoming to a high of 7,807 in California. On average, there are 1,693 respondents per state, thus decreasing the likelihood of measurement error in the independent variable. 6 My data allow for an accurate assessment of state opinion, especially when broken into terciles.
The summary statistics for the estimated constituency opinion are available in online Appendix B. Although the mean of state opinion for each group tends to be conservative, Lower Income Constituency Opinion is, on average, the most liberal of each of the three groups with a mean of 0.129. Both Middle-Income Constituency Opinion and Upper Income Constituency Opinion have higher values, indicating they are (on average) more conservative than groups on the lower income strata. Interestingly, Middle-Income Opinion is, on average, the most conservative of the three groups. 7 These differences in constituency opinion indicate that Senators are faced with choices in terms of who they will be responsive to, as different class subconstituencies often have competing ideological orientations. Although the average constituent in each income group leans toward conservative, the summary statistics show that there is a wide variation in state ideology. This presents legislators with a dilemma in many cases, as there are instances in which they must choose whether to respond to one group over another.

Testing Unequal Responsiveness
In this study, as in other dyadic studies of representation, I measure responsiveness as the correspondence between legislator behavior and constituency opinion. To test the hypotheses I developed in the previous section, I use measures of legislator ideology, party membership, and constituency ideology. I estimate a standard ordinary least squares (OLS) regression analysis to examine the degree to which legislators respond to different income groups. 8 Legislator ideology is measured by using DW-NOMINATE scores, which are a measure developed to summarize a legislator's ideological position based on all the votes they cast (Poole and Rosenthal 1997). 9 The scale for these scores range from −1 for the most liberal Senators to 1 for the most conservative. 10 Summary statistics for this variable (available in online Appendix B) show that four of the five Congresses were more likely to be conservative, but only by a slight margin. Most of the Congresses obtain a mean right in the middle of the −1 to 1 scale. However, each Congress differs in terms of the standard deviation and degree to which extreme liberals or conservatives are present. This allows for a unique opportunity to observe responsiveness to constituency opinion during a period of five consecutive Congresses. I examine responsiveness by looking at the relationship between legislator behavior and ideology by income group (within each state). The results for the OLS regression analysis are displayed in Table 1. Each column of the table displays a standard OLS regression analysis (for each separate Congress), where the dependent variable is Legislator Ideology measured using first dimension DW-NOMINATE scores. 11 The independent variables reflect Constituency Conservatism measured using data from the 2004 NAES, which asked respondents to place themselves on a traditional ideological scale. 12 Therefore, each column shows the degree to which Senators are responsive to the opinion of different income groups. 13 The results are quite striking in that Senators are found to be positively and significantly responsive to ideology of upper income constituents in each of the five Congresses. 14 I am unable to detect responsiveness to either middle-or lower income constituency opinion.
The substantive significance of the results in Table 2 can be seen by looking at the graphical display of the regression estimates for each of the five Congresses under examination. As Figure 2 demonstrates, Senators are found to be positively responsive to upper income constituents, and either only slightly or negatively responsive to lower income constituents or middleincome constituents.
The nature of unequal responsiveness can be seen in Figure 3, which displays the predicted responsiveness for Senators in the 110th Congress to each income tercile. In the figure, the dotted line represents the predicted values, whereas the shaded area shows the 95 percent confidence interval. 15 As the figure quite clearly shows, the predicted responsiveness for both lower constituency opinion and middle constituency opinion is flat, whereas the responsiveness toward upper income constituency opinion is positive.    In addition to examining a basic model of responsiveness, I control for two important variables. An additional control variable measures the percentage of African Americans in a state. This control is included due to the fact that the proportion of African Americans in a district likely has little impact on Republican legislator voting behavior as this group votes cohesively for the Democratic Party. In effect, including the percentage of African Americans in a state is a way to distinguish minority poor from nonminority poor. Moreover, I control for the effects of party as well. The results from the OLS regression controlling for party are displayed in Table 3. This table shows responsiveness of Senators of the 107th through 111th Congresses toward constituent ideology (separated by income group). Each column of the table displays a standard OLS regression analysis (for each separate Congress), where the dependent variable is Legislator Ideology measured using first dimension DW-NOMINATE scores. Each column shows the regression estimate for the independent variables of constituency opinion with a control variable for each state percentage of African Americans (percentage black) and whether a Senator is a member of the Republican Party. 16 The results displayed in Table 2 are quite striking when examining the degree to which Senators respond unequally to different economic subconstituencies, even after controls are included. Most noticeably, this table demonstrates that Senators are consistently responsive to upper income constituents as the Upper Income Opinion variable is both positive and statistically significant in all five Congresses. 17 Turning to the other constituency opinion variables, I am able to detect responsiveness by Senators to Middle-Income Opinion in two of the five Congresses, as this variable is both positive and statistically significant in the 110th and 111th Congresses. As each of these Congresses is controlled by the Democrats, I find partial support for the Agenda Control Hypothesis, at least to the extent that groups besides the wealthy receive representation when Democrats have majority control of the chamber. However, the independent variable measuring Low-Income Opinion does not reach statistical significance in any Congress, demonstrating that I am unable to detect responsiveness to this income group. Moreover, Low-Income Opinion is negative in each of the five Congresses, demonstrating that Senators seem to act in the opposite direction of the interests of this group. 18 As expected, the variable Republican retains a positive and statistically significant coefficient in all five Congresses, indicating Senators' party affiliations are strongly related to their voting patterns.
The substantive results of all the findings in Table 2 are displayed in Figure 4. This figure shows the effect of differential responsiveness across Congresses, as the regression estimate is detailed for each income group as a bar on the graph. Clearly, the responsiveness estimate of upper income opinion is much larger than either low-or middle-income opinion in most Congresses, but especially when examining the 107th, 108th, and 109th. Only in the 110th and 111th Congress is the estimate for middle-income opinion higher than that for upper income opinion.
Again, some support for the Agenda Control Hypothesis is present, as the Democratic Party having a majority of seats in the Senate does appear to allow for the responsiveness of groups besides the wealthy. However, in no case is there positive responsiveness toward the poor, and thus this hypothesis receives only minimal support. These results add support to my initial expectation that Senators are more likely to respond to the preferences of upper income constituents. Therefore, the Unequal Responsiveness Hypothesis receives strong support from the results.

Partisan Differences in Responsiveness
Although the preceding results show that responsiveness most clearly occurs for upper income constituents, it is an open question whether members of one of the two major political parties are more responsive to certain economic groups than others. I test this possibility by creating an interaction by multiplying the Republican variable (0 = Democrat, 1 = Republican) by income group opinion and adding that variable to the model used in the previous section. This allows me to examine the independent effect that party has on responsiveness toward each income group. The results to the interactive models for each Congress are shown in Table 3.   The key variables of interest are the interaction terms for each income group at the bottom of each column. In the 109th Congress, the interaction variable GOP × Middle Income Opinion is positive and significant. This means that Republicans are more responsive than Democrats to the middle class in this Congress. 19 These results run counter to my initial expectation that Democrats would be more responsive to lower income groups and that Republicans would be more responsive to upper income groups. Instead, the single finding of a partisan bias in responsiveness comes in the form of Republicans responding to the desires of the middle class more than Democrats in the 109th. Therefore, the Partisan Bias Hypothesis (as previously outlined) does not receive support from these results.
As responsiveness toward the upper income opinion is strong in each of the Congresses under examination, I am unable to detect much support for the Agenda Control Hypothesis. In fact, responsiveness toward the wealthy under democratically controlled Congresses is equal to that of Republican-controlled Congresses. Moreover, as the variable measuring lower income opinion does not achieve statistical significance under any Congress, it does not seem that a greater amount of responsiveness toward these groups is present when the Senate is controlled by either party.

Testing Agenda Control: The 107th Senate
The 107th Senate presents an interesting and rare opportunity to test the degree to which agenda control effects, the way in which a change in the nature of issues on the agenda (and which party control the agenda), can influence the way in which legislators respond to different income groups. At the beginning of the 107th Senate session, the Republican Party had unified control of the government, as George W. Bush's victory in the 2000 presidential election handed the party control of the presidency, and both chambers of the Congress. Although the Senate was effectively split evenly between the parties with fifty members from each party elected, Republican control of the presidency gave Vice President Dick Cheney the tie-breaking vote in the Senate, and thus majority control of the chamber.
The Bush administration wasted little time in pushing for tax cuts. Passed first in 2001Passed first in (and later in 2003, the Bush tax cuts were some of the largest tax cuts in history and largely benefited the wealthy (Bartels 2008). Senator Jim Jeffords (R-VT) was a key moderate in the negotiations and was a vote for which both parties courted in the many efforts to reach a compromise on the size of the tax cuts. Although Jeffords voted for the final version of the tax cut in 2001, his patience with the president and the lack of moderates in the Republican Party had grown thin. On May 26, 2001, citing disagreements over abortion, education, and taxes, Jeffords announced he was changing his party affiliation to an Independent and would caucus with the Democrats, thus handing them control of the chamber. 20 In his speech announcing his decision, Jeffords said that in dealing with party leaders and the president, it was difficult, "to deal with me and for me to deal with them . . . Looking ahead, I can see more and more instances in which I will disagree with the president on fundamental issues" (Margasak 2001).
Jeffords' switch presents an interesting most similar (within) case study to test the effects of a change in partisan control on legislator responsiveness toward different income groups. 21 This is an opportunity in which we can examine the same group of legislators during a similar time period in which partisan control of the agenda changes. This presents an excellent opportunity for a most similar case-study analysis (e.g., Gerring 2007).
To test the Agenda Control Hypothesis, I estimated W-NOMINATE scores using the W-NOMINATE program in R for Senators both before and after Jeffords switched parties. The Jeffords' switch does have an effect on the agenda, as the W-NOMINATE scores for the period before the switch are, on average, more conservative than in the period after the switch. In fact, there is a total change in the average W-NOMINATE score of −.186, which is a statistically significant difference (see Table 5b in online Appendix B).
It is not the ideology of members that changed in the 107th Senate after Jeffords switched parties, but that the issues on the agenda. As Keith Poole (2003) writes, "Members of Congress die in their ideological boots. That is, based on the roll call voting record, once elected to Congress, members adopt an ideological position and maintain that position throughout their careers." As it is highly unlikely that Senators collectively decided to become more liberal, the change in W-NOMINATE scores is likely the result of issues on the agenda changing, which would likely be more liberal with Democrats controlling the chamber. Because there is an observed shift in the agenda, I run similar analyses (both before and after Jeffords switched) as in previous sections to determine which groups receive responsiveness from Senators and whether there are partisan differences in this responsiveness.
The results to multiple OLS regression analyses are reported in Table 4. In the first three columns, we can see the results for Senator responsiveness before Jeffords' party switch without controls (Column 1), controlling for party and percentage African American (Column 2), and with interactions to test differences in partisan responsiveness (Column 3). The first two models show similar results as before; Senators are responsive to upper income constituents whether controlling for party or not. In the third model, none of the interaction variables achieve statistical significance, meaning that we are unable to see partisan differences in responsiveness before Jeffords switched parties.
The second half of Table 4 reports the results after Jeffords switches to an Independent, and the Democrats took control of the Senate. 22 Once again, three different models are presented: one without controlling for party (Column 4), another controlling for party (Column 5), and a third model with interactions to test differences in partisan responsiveness (Column 6). Once again, Senators are responsive to their upper income constituents, yet I am not able to detect responsiveness to either low-or middle-income opinion. In fact, overall responsiveness toward upper income constituency opinion is greater once Democrats take control of the chamber as the coefficient is larger after Jeffords' switch. In the final column of the table, none of the interaction terms are statistically significant, meaning that I am unable to detect evidence that either party is more responsive than the other. In sum, these findings show that the poor do seem to receive greater responsiveness under Democratic control of the agenda. Neither party appears to be more responsive toward the wealthy after Jeffords' switch.

Discussion
I find evidence of responsiveness to the wealthiest constituents in each of the Congresses I examine, some responsiveness to middle-income constituents in two Congresses, and no detectable responsiveness to lower income groups in any Congress. Of all the hypotheses I tested, the strongest and most consistent support was with the Unequal Representation Hypothesis, as those in the upper class received responsiveness by Senators in each Congress I examined. The nature of this unequal responsiveness differs from those of Bartels (2008). Bartels found responsiveness of Senators toward middle-income constituents. I only find this to be the case in two of the five Congresses under examination. In three of the Congresses (107th-109th), the results show responsiveness solely to upper income constituents. Moreover, although Bartels finds Republicans to be more responsive to the preferences of upper income constituents, this analysis does not find evidence that either party is more responsive to this group. In only one instance (109th) do Ordinary least squares regression coefficients with standard errors in parentheses (clustered by state) for Poole-Rosenthal's W-NOMINATE scores (before and after Jeffords' party switch). *p < .10. **p < .05. ***p < .01 (two-tailed tests).
I detect evidence that one party is more responsive to middle-income constituents. Finally, by incorporating the case study of the 107th Senate, I am able to examine the effects of agenda control. As my results differ from Bartels (2008), it seems to be the case that unequal responsiveness is now more pronounced than in previous decades. This change in responsiveness could reflect the growing inequality in America (e.g., Piketty and Saez 2003) or perhaps increasing polarization in Congress (e.g., McCarty, Poole, and Rosenthal 2006). Unequal responsiveness could also be the result of campaign contributions and the fact that this form of political participation is dominated by the wealthy (e.g., Verba, Schlozman, and Brady 1995).
As for the Partisan Bias Hypothesis, the evidence is quite mixed. I do find greater responsiveness by Democrats to the wealthy in the 111th Congress. In addition, I find evidence that the Republican Party is more responsive than Democrats to middle-income constituents in one of the Congresses analyzed (109th). The case study of the 107th Congress does not provide evidence of greater responsiveness by the Democratic Party to the poor once they control the agenda. Instead, once the Democratic Party takes control of the chamber, responsiveness to the wealthy increases. Moreover, in each Congress examined, the Democrats were no more responsive to the poor than were the Republicans. This might be due to the fact that a filibuster-proof majority is a rare occurrence, which is increasingly an important tool in American governance in terms of the Senate (e.g., Koger 2010). Alternatively, the fact that the Democratic Party does not seem to respond better to lower income groups could reflect a change in distributional goals of a party once set on helping the disadvantaged in American society. The fact that lower income groups seem to be ignored by elected officials, although not a new finding (e.g., Bartels 2008;Gilens 2005), remains a troubling observation in American politics.
One possible explanation for the differences in responsiveness we see over time lies with differences in partisan control of Congress. However, the results presented in this analysis do not suggest this to be the case. Lowincome constituents were not better represented once Jeffords switched parties and handed control of the agenda to the Democrats in the 107th Congress. In addition, I was unable to detect responsiveness to this group when the Democrats controlled the Senate during the 110th and 111th sessions. The results show about the same level of (and in some cases more) responsiveness to upper class opinion under democratically controlled Senates. Part of the explanation for this finding might be due to the fact that agenda control matters much more in the House than in the Senate.
My findings suggest that Senators do not respond to the views of all their constituents in an equal manner. My model of responsiveness assumed that Senators are equally adept at discerning the views of each income group constituency, which is not likely to occur in the real world, especially given the fact that different groups are more intense about some issues than others (e.g., Bishin 2009). Senators could be responding to what they think the views of each constituency are, which is likely to lead to erroneous judgments or biased responsiveness toward upper income groups, especially when the average Senator is not likely to come from the same socioeconomic class as those citizens at the bottom of the economic rung (e.g., Carnes 2012).

Conclusion
This article examined the nature of unequal responsiveness in the U.S. Senate in an era for which wealth and income inequality has grown exponentially. I find strong support for unequal responsiveness in the Senate during the period of the 107th through 111th Congresses. For multiple issues, and across a number of recent Congresses, the results differ from previous findings regarding biased responsiveness, most specifically the findings of Larry Bartels (2008), who found responsiveness to middle-and upper income groups. My analysis, which examines Senator behavior on a large number of votes, shows evidence of responsiveness to only the wealthy, a distinct problem for any democracy. In some ways, this suggests oligarchic tendencies in the American system, a finding echoed in other research (e.g., Winters and Page 2009). The results consistently show that those with more means have, at the very least, a thumb on the scale when it comes to responsiveness. The results do not show the same for those at the bottom of the economic ladder.
My findings are a call for more research in the area of wealth inequality and representation. There has been a tradition in the study of American politics of treating economic class (e.g., Income) as a control variable, rather than as something more. This largely stems from the idea that Americans do not identify along class lines, at least not to the extent as citizens in European democracies. My results caution against this traditional approach, as I consistently find a clear bias in responsiveness toward the wealthy. Although Americans might not easily identify along class lines, this does not mean that politicians representing these citizens do not respond to them in this manner. If equal responsiveness is a fundamental practice in a democratic society (e.g., Dahl 1971), my findings question the degree to which this occurs.
In addition, the matter of issue intensity, especially among different income groups, may play a large role in my findings. However, I was not able to measure issue intensity and therefore my findings are a call for more research into subconstituency politics, as it matters which types of group parties are responsive to as different groups care about some issues more than others (e.g., Bishin 2009). Moreover, although it is clear that Americans often have conflicting views on such matters relating to inequality (e.g., McClosky and Zaller 1984), more needs to be done to examine the circumstances under which people care about reducing inequality, especially when different groups benefit and care about issues related to inequality related in different ways.
Although I do find consistent responsiveness of Senators to the upper class, it is worth considering whether the Senate is functioning just as the Founders intended. It was the House of Representatives that was designed to be the "people's branch" and to represent the interests of the majority. The Senate, in contrast, was set up to control popular excesses. Originally elected by the state legislatures, Senators were meant to be insulated from the masses. Because of this, future research must examine the extent to which different income groups receive different types of representation in the House of Representatives. Nevertheless, unequal responsiveness should be an area that receives great scrutiny from political scientists in the future, as our era is likely to be defined by the great wealth inequality not seen since the first Gilded Age.

Acknowledgments
The author would like to thank Benjamin Bishin, Jamil Bilem, Leigh Bowles, Christopher Dennis, Bernie Grofman, Martin Johnson, Antoine Yoshinaka, and students at the research colloquium at University of California, Riverside, for comments on earlier drafts.

Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article. significantly underrepresented many groups of citizens, such as young people, racial and ethnic minorities, and people with little formal education. This is especially problematic as the underrepresentation of these opinions on matters relating to economic inequality could lead to biased estimates. To account for this, as well as to better estimate state opinion, I post-stratified the sample within each state on the basis of education, sex, race, and age. 4. I tried to both create groups that were relatively even in number in terms of respondents as well as matching a concept of class that was appropriate for this study. My classification differs somewhat from the previous literature (Bartels 2008). Much of this difference is due to the categories of income made available by the NAES as well as the fact that I wanted to have relatively equal numbers of respondents in each grouping. I also tried other income groupings of respondents; however, the results of these analyses were largely similar to those reported in this article. 5. As Stimson (2004) finds that many individuals who identify as conservative are operationally liberal (e.g., favor government spending), an alternative way to measure constituency would be to use a measure that includes attitudes toward government spending on various programs. However, as the 2004 NAES is limited in the number of questions asked about favorability toward government spending programs, I am unable to construct such a measure here. 6. In fact, this is a key instance in which the data that I use are superior to those used by Bartels (2008). To estimate state opinion, Bartels used the Senate Election Study, which had an average of approximately 185 respondents per state. 7. Difference in mean tests among groups (t-tests) shows that the lower and upper opinion groups are statistically different from one another (as is the difference between lower and middle). The difference between middle and upper is not statistically different, largely because the average ideological opinion of each group is conservative. Nonetheless, Senators often face a choice within their states, as middleincome constituency opinion and upper income constituency opinion often differ as well. 8. The basic notation is similar to Bartels' (2008)

Notes
where Y k is an observed roll call vote (or summary of votes) cast by Senator k; P k is the percentage of poor-, middle-, or upper income constituents from Senator k's state; X is the opinion of a specific survey respondent i; (βX i ) is the weight attached to a respondent's opinion; е is a stochastic term representing other influences on a Senator k's behavior; and α, β, and γ are parameters to be estimated. The key parameter to be estimated is β, which measures the statistical relationship between Senator behavior (e.g., roll call voting) and constituency opinion. As this method of estimation produces three distinct income groups, no baseline category is needed and all three may be included in the model. Moreover, my model differs from Bartels in that I multiply respondent opinion by the percentage of poor-, middle-, or upper income constituents in each state (based on 2000 Census). In effect, this controls for the size of the income group population (poor, middle, and upper) in each state. 9. Bartels (2008) uses W-NOMINATE scores to test responsiveness of Senators to constituency preferences. As W-NOMINATE scores are static and not directly comparable between Congresses (see http://www.voteview.com/page2a. htm), I use DW-NOMINATE scores for my dependent variable. I also estimated W-NOMINATE scores using the W-NOMINATE Roll Call Analysis Software for R. The results (available in online Appendix B) are similar to those presented here. In reality, W-NOMINATE and DW-NOMINATE scores correlate at an extremely high rate and therefore they are comparable measures of legislator ideology (Poole and Rosenthal 1997). 10. Summary statistics for the dependent and independent variables can be found in online Appendix B. 11. Because the two voting record observations of each Senator in a given state might not be independent, standard errors are clustered by state. 12. All question wording and variable coding can be found in online Appendix A. 13. To control for the size of each group by state, I use Census (2000) data, and multiply the average of each income group constituency opinion by the size of the population in each state. 14. The correlation between the income variables is actually quite high, which can be seen in online Appendix B. Despite this high correlation, I generally find responsiveness toward upper income constituents, demonstrating that the impact of upper opinion is quite substantial. 15. To estimate predicted values, I used the margins program in STATA, which estimates predicted values while holding all other variables at their means. 16. I coded all Senators identifying themselves as Independents as either Republicans or Democrats according to the party they caucused with. The number of cases varies between Congresses due to Senator retirement or illness. 17. Although many of the estimated coefficients of low-and middle-income constituency opinion are negative, the estimated impact is too small in both absolute terms and in comparison with the standard error. This means that I am unable to rule out zero as a plausible value and can therefore not say with certainty that Senators are negatively responsive, only that I am unable to detect responsiveness by Senators to these groups. 18. Tests of equality (online Appendix B) show that the difference in coefficients between lower and upper income opinion is significantly different from each other in each model. Moreover, the difference in coefficients between lower income opinion and middle-income opinion is significant in three of the five models (109th, 110th, and 111th). These tests, along with an examination of the size and magnitude of each coefficient allows for a sense of the degree to which each group receives responsiveness that is more or less than other groups (at least for those that are significantly different from one another). For example, the coefficient for upper income opinion is larger than the lower income opinion coefficients in all Congresses, indicating that not only are Senators responsive to the wealthy in each Congress but significantly more so than to the poor. 19. The fact that high-income respondents in richer states vote more Democratic than high-income respondents in poorer states (e.g., Gelman et al. 2008) might explain this finding. However, a model that includes an interaction between median household income and Senator party affiliation does not show this to be the case. These results are available on request. 20. Despite announcing in late May, Jeffords promised to delay his switch until negotiations for the tax cuts had finalized, and thus began to caucus with the Democrats on June 6, 2001. 21. The switch by Jeffords falls short of meeting the requirements for a natural experiment, in that the change is not due to an exogenous treatment effect as Jeffords himself causes the change in agenda control (see Sekhon and Titiunik 2012). 22. The changes in representation that are associated with the Democratic takeover of the Senate could be due to some combination of agenda control as well as differences in the preferences of the newly elected Senators. This appears unlikely, however, as the results remain similar if freshmen are excluded from the analysis (these results appear in Table  5b of online Appendix B).