PHILADELPHIA –- Why is there so much disagreement about whether using recreational drugs is morally wrong? A University of Pennsylvania psychology study shows that the debate about drugs might really be about sex.

The study compared two competing theories.

One theory -- the conventional wisdom in political science -- sees drug attitudes as primarily coming from people's political ideology, level of religious commitment, and personality, for example, openness to experience. 

The other theory, proposed by the researchers and driven by ideas from evolutionary psychology, holds that drug attitudes are really driven by people's reproductive strategies.

When the Penn researchers questioned almost 1,000 people in two subject populations, one undergraduate and one Internet-based, a clear winner emerged between the competing theories:  Differences in reproductive strategies are driving individuals' different views on recreational drugs.

While many items predict to some extent whether people are opposed to recreational drugs, the most closely related predictors are people's views on sexual promiscuity.  While people who are more religious and those who are more politically conservative do tend to oppose recreational drugs, in both study samples the predictive power of these religious and ideological items was reduced nearly to zero by controlling for items tracking attitudes toward sexual promiscuity.

“This provides evidence that views on sex and views on drugs are very closely related,” said Robert Kurzban, associate professor in the Department of Psychology and director of the Pennsylvania Laboratory for Experimental Evolutionary Psychology at Penn.  “If you were to measure people's political ideology, religiosity and personality characteristics, you can predict to some degree how people feel about recreational drugs.  But if, instead, you just measure how people feel about casual sex, and ignore the abstract items, the predictions about people's views on drugs in fact become quite a bit better.” 

From a theoretical perspective, the study also concludes that considering morality from the standpoint of strategic reproductive interests is a potentially useful way to understand why humans care about third-party behavior.

According to the researchers' evolutionary model, people develop complex differences in their sexual and reproductive strategies.  One key difference that creates strategic conflict arises in people's orientations towards casual sexual activity.  The relationships of people following a more committed, monogamous reproductive strategy are put at greater risk when casual sex is prevalent.  On the other hand, people pursuing a less committed lifestyle seek to avoid having their choices moralized, forbidden and punished.

The researchers cite prior work showing that recreational drug usage is often associated with promiscuity.  The results of the study imply that attitudes against recreational drugs are part of a larger attempt to advance the cause of committed, monogamous reproductive strategies.

“Condemnation of drug usage might be best understood in the context of strategic dynamics, with individuals influencing moral rules in a way that favors their own competitive reproductive strategies,” Kurzban said.  “We expect that this relationship between sexual strategy and moral stances will occur in other areas as well, such as attitudes toward prostitution, sexual education or abortion.”    

The research team analyzed questionnaires from 516 undergraduate students from the University of Central Florida and 471 individuals recruited from a Web-based recruitment site, Amazon’s Mechanical Turk, or Mturk.

Participants reported their overall liberal/conservative political identification, rated their support or opposition to a number of current political issues and answered questions about their personalities, disgust sensitivity, moral views, sexual attitudes and level of religiosity.  The measure of recreational drug attitudes consisted of questions on the morality and legal status of using marijuana, cocaine and Ecstasy, as well as general attitudes towards recreational drugs.

The study, published in the current issue of Proceedings of the Royal Society B, was conducted by Kurzban of Penn’s Department of Psychology in the School of Arts and Sciences, Jason Weeden of the Pennsylvania Laboratory for Experimental Evolutionary Psychology at Penn and Amber Dukes of the University of Central Florida.