Conservatives exhibit less cognitive ability than liberals do. Or that's what it says in the social science literature, anyway. A 2010 study using data from the National Longitudinal Study of Adolescent Health, for example, found that the IQs of young adults who described themselves as "very liberal" averaged 106.42, whereas the mean of those who identified as "very conservative" was 94.82. Similarly, when a 2009 study correlated cognitive capacity with political beliefs among 1,254 community college students and 1,600 foreign students seeking entry to U.S. universities, it found that conservatism is "related to low performance on cognitive ability tests." In 2012, a paper reported that people endorse more conservative views when drunk or under cognitive pressure; it concluded that "political conservatism may be a process consequence of low-effort thought."

So have social scientists really proved that conservatives are dumber than liberals? It depends crucially on how you define "conservative."

For an inkling of what some social scientists think conservatives believe, parse a 2008 study by the University of Nevada at Reno sociologist Markus Kemmelmeier. To probe the political and social beliefs of nearly 7,000 undergraduates at an elite university, Kemmelmeier devised a set of six questions asking whether abortion, same-sex marriage, and gay sex should be legal, whether handguns and racist/sexist speech on campus should be banned, and whether higher taxes should be imposed on the wealthy. The first three were supposed to measure the students' views of "conservative gender roles," and the second set was supposed to gauge their "anti-regulation" beliefs. Kemmelmeier clearly thought that "liberals" would tend to be OK with legal abortion, same-sex marriage, and gay sex, and would opt to ban handguns and offensive speech and to tax the rich. Conservatives would supposedly hold the opposite views.

Savvy readers may recognize a problem with using these questions to sort people into just two ideological categories. And sure enough, Kemmelmeier got some results that puzzled him. He found that students who held more traditional views on gender and sex roles averaged lower on their verbal SAT and Achievement Test scores. "Surprisingly," he continued, this was not true of students with anti-regulation attitudes. With them, "all else being equal, more conservative respondents scored higher than more liberal respondents." Kemmelmeier ruefully notes that "this result was not anticipated" and "diametrically contradicts" the hypothesis that conservatism is linked to lower cognitive ability. Kemmelmeier is so evidently lost in the intellectual fog of contemporary progressivism that he does not realize that his questionnaire is impeccably designed to identify classical liberals, a.k.a. libertarians, who endorse liberty in both the social and economic realms.

So how smart are libertarians compared to liberals and conservatives? In a May 2014 study in the journal Intelligence, the Oxford sociologist Noah Carl attempts to answer to that question. Because research has "consistently shown that intelligence is positively correlated with socially liberal beliefs and negatively correlated with religious beliefs," Carl suggests that in the American political context, social scientists would expect Republicans to be less intelligent than Democrats. Instead, Republicans have slightly higher verbal intelligence scores (2–5 IQ points) than Democrats. How could that be?

Carl begins by pointing out that there is data suggesting that a segment of the American population holding classical liberal beliefs tends to vote Republican. Classical liberals, Carl notes, believe that an individual should be free to make his own lifestyle choices and to enjoy the profits derived from voluntary transactions with others. He proposes that intelligence actually correlates with classically liberal beliefs.

To test this hypothesis, Carl uses data on political attitudes and intelligence derived from the General Social Survey, which has been administered to representative samples of American adults every couple of years since 1972. Using GSS data, respondents are classified on a continuum ranging from strong Republican through independent to strong Democrat. Carl then creates a measure of socially liberal beliefs based on respondents' attitudes toward homosexuality, marijuana consumption, abortion, and free speech for communists, racists, and advocates for military dictatorship. He similarly probes liberal economic views, with an assessment of attitudes toward government provision of jobs, industry subsidies, income redistribution, price controls, labor unions, and military spending. Verbal Intelligence is evaluated using the GSS WORDSUM test results.

Comparing strong Republicans with strong Democrats, Carl finds that Republicans have a 5.48 IQ point advantage over Democrats. Broadening party affiliation to include moderate to merely leaning respondents still results in a Republican advantage of 3.47 IQ points and 2.47 IQ points respectively. Carl reconciles his findings with the social science literature that reports that liberals are more intelligent than conservatives by proposing that Americans with classically liberal beliefs are even smarter. Carl further reports that those who endorse both social conservatism and economic statism also have lower verbal IQ scores.

"Overall, my findings suggest that higher intelligence among classically liberal Republicans compensates for lower intelligence among socially conservative Republicans," concludes Carl. If the dumb, I mean socially conservative, Republicans keep disrespecting us classical liberals, we'll take our IQ points and go home.

As gratifying as Carl's research findings are, it is still a deep puzzle to me why it apparently takes high intelligence to understand that the government should stay out of both the bedroom and the boardroom.