The relevant article was recently published, although the entire thing is behind a subscription wall, so the authors published a summary (much more detailed than a traditional "abstract") here. The whole thing is worth reading, and it isn't very long.

Some highlights:

In this article, we suggest that one largely overlooked cause of failure is a lack of political diversity. We review evidence suggesting that political diversity and dissent would improve the reliability and validity of social psychological science…

We focus on conservatives as an underrepresented group because the data on the prevalence in psychology of different ideological groups is best for the liberal-conservative contrast – and the departure from the proportion of liberals and conservatives in the U.S. population is so dramatic.

The lack of diversity causes problems for the scientific process primarily in areas related to the political concerns of the Left – areas such as race, gender, stereotyping, environmentalism, power, and inequality – as well as in areas where conservatives themselves are studied, such as in moral and political psychology.

It's refreshing to see a group of maverick academics admit that, in effect, many of the studies of the sharpest interest between left and right (and which, of course, are the most salient for the issues discussed at PPD) are at risk of being inherently biased in favor of the orthodox left. This is something that has been intuitive to any of who are both not members of the orthodox left and who have spent some time around the humanities/social sciences academy, but it is nevertheless refreshing to see a small group of academics admit to this, and have the balls both to do that and to study how and why that is the case.

More:

Before the 1990s, academic psychology only LEANED left. Liberals and Democrats outnumbered Conservatives and Republican by 4 to 1 or less. But as the “greatest generation” retired in the 1990s and was replaced by baby boomers, the ratio skyrocketed to something more like 12 to 1. In just 20 years. Few psychologists realize just how quickly or completely the field has become a political monoculture. This graph took us by surprise too.

Might a shared moral-historical narrative [the “liberal progress” narrative described by sociologist Christian Smith] in a politically homogeneous field undermine the self-correction processes on which good science depends? We think so, and present three risk points— three ways in which political homogeneity can threaten the validity of social psychological science

3.1. Risk point 1: Liberal values and assumptions can become embedded into theory and method The embedding of values occurs when value statements or ideological claims are wrongly treated as objective truth, and observed deviation from that truth is treated as error.

[Example:] Son Hing, Bobocel, Zanna, and McBride (2007) found that: 1) people high in social dominance orientation (SDO) were more likely to make unethical decisions, 2) people high in right-wing authoritarianism (RWA) were more likely to go along with the unethical decisions of leaders, and 3) dyads with high SDO leaders and high RWA followers made more unethical decisions than dyads with alternative arrangements (e.g., low SDO—low RWA dyads). Yet consider the decisions they defined as unethical: not formally taking a female colleague’s side in her sexual harassment complaint against her subordinate (given little information about the case), and a worker placing the well-being of his or her company above unspecified harms to the environment attributed to the company’s operations. Liberal values of feminism and environmentalism were embedded directly into the operationalization of ethics, even to the extent that participants were expected to endorse those values in vignettes that lacked the information one would need to make a considered judgment. The appearance of certain words that imply pernicious motives (e.g., deny, legitimize, rationalize, justify, defend, trivialize) may be particularly indicative of research tainted by embedded values.

3.2. Risk point 2: Researchers may concentrate on topics that validate the liberal progress narrative and avoid topics that contest that narrative.

Since the enlightenment, scientists have thought of themselves as spreading light and pushing back the darkness. The metaphor is apt, but in a politically homogeneous field, a larger-than-optimal number of scientists shine their flashlights on ideologically important regions of the terrain. Doing so leaves many areas unexplored. Even worse, some areas become walled off, and inquisitive researchers risk ostracism if they venture in.

3.3. Risk point 3: Negative attitudes regarding conservatives can produce a psychological science that mischaracterizes their traits and attributes

These seem clear enough, to anyone who doesn't toe the orthodox progressive line. Yet, the question remains -- why is the situation this way? Haidt and colleagues looked into this as well:

  1. Why are there so few non-liberals in social psychology? the evidence does not point to a single answer. To understand why conservatives are so vastly underrepresented in social psychology, we consider five explanations

5.1. Differences in ability [Are conservatives simply less intelligent than liberals, and less able to obtain PhDs and faculty positions?] The evidence does not support this view… [published studies are mixed. Part of the complexity is that…] Social conservatism correlates with lower cognitive ability test scores, but economic conservatism correlates with higher scores (Iyer, Koleva, Graham, Ditto, & Haidt, 2012; Kemmelmeier 2008). [Libertarians are the political group with the highest IQ, yet they are underrepresented in the social sciences other than economics]

5.2. The effects of education on political ideology Many may view education as “enlightening” and believe that an enlightened view comports with liberal politics. There is little evidence that education causes students to become more liberal.

5.3. Differences in interest Might liberals simply find a career in social psychology (or the academy more broadly) more appealing? Yes, for several reasons.

5.4. Hostile climate Might self-selection be amplified by an accurate perception among conservative students that they are not welcome in the social psychology community?

Evidence of hostile climate is not just anecdotal. Inbar and Lammers (2012) asked members of the SPSP discussion list: “Do you feel that there is a hostile climate towards your political beliefs in your field?” Of 17 conservatives, 14 (82%) responded “yes” (i.e., a response at or above the midpoint of the scale, where the midpoint was labeled “somewhat” and the top point “very much”), with half of those responding “very much.” In contrast, only 18 of 266 liberals (7%) responded “yes”, with only two of those responding “very much.” Interestingly, 18 of 25 moderates (72%) responded “yes,” with one responding “very much.” This surprising result suggests that the hostile climate may adversely affect not only conservatives, but anyone who is not liberal or whose values do not align with the liberal progress narrative.

5.5. Discrimination The literature on political prejudice demonstrates that strongly identified partisans show little compunction about expressing their overt hostility toward the other side (e.g., Chambers et al., 2013; Crawford & Pilanski, 2013; Haidt, 2012). Partisans routinely believe that their hostility towards opposing groups is justified because of the threat posed to their values by dissimilar others (see Brandt et al., 2014, for a review). Social psychologists are unlikely to be immune to such psychological processes. Indeed, ample evidence using multiple methods demonstrates that social psychologists do in fact act in discriminatory ways toward non-liberal colleagues and their research.

Conservative graduate students and assistant professors are behaving rationally when they keep their political identities hidden, and when they avoid voicing the dissenting opinions that could be of such great benefit to the field. Moderate and libertarian students may be suffering the same fate.

-- What is the relevance of this to PPD?

That should be clear.

Why on Earth should those of us who dissent from the liberal/progressive orthodoxy on these issues that we discuss here at PPD (which are in the scope of what the researchers here found to be most subject to the kinds of biases they address) defer to peer-reviewed papers coming from an academy that systemically discriminates against alternative points of view and which, in many ways, acts as an echo chamber of liberal progressive orthodoxy? This is, in fact, the main reason I have never laid a lot of stock in peer-reviewed social science research -- it is hopelessly biased, due to the overwhelming biases of the people conducting it. Thankfully, there are a few academics of the left who see the problems in having their side be hegemonic in the academy, such that this research is taking place. But until some kind of balance is restored, substantial skepticism understates the proper approach to be taken to research being done in these "politically and socially sensitive" areas. Something to keep in mind when citing, or responding to a cite, of this or that peer-reviewed social science "study".