Prior research suggests that people who condemn drug use over moral grounds also tend to judge others harshly who engage in promiscuous, non-monogamous sex. A new study that involved more than 8,000 twins not only confirmed this link but also showed the association may be mediated by genes. Those who wrap their negative views regarding sexuality and drug use in a veneer of morality may, deep down, actually be looking out for their own reproductive strategy by shaming others in order to control the environment.

Public condemnation of casual sex and illicit drug use has never really gone away, despite massive cultural shifts during the 1960s counterculture movement. Although upbringing certainly has a part to play in shaping one’s views of the world and moral compass, psychologists have amassed increasing evidence that many of the instances when we righteously point our fingers may be selfish acts of self-interest.

It’s common for people who disapprove of illicit drug use to also frown upon casual sex. Each of these instances shouldn’t bother other people since it doesn’t affect them directly in any way unless they interact with people who engage in them. But past studies have shown that openness to engage in casual sex is partially explained by genes. And those who are inclined to engage in noncommittal sex are also more likely to use recreational drugs.

“People adopt behaviors and attitudes, including certain moral views, that are advantageous to their own interests. People tend to associate recreational drug use with noncommitted sex. As such, people who are heavily oriented toward high commitment in sexual relationships morally condemn recreational drugs, as they benefit from environments in which high sexual commitment is the norm,” said Annika Karinen, a researcher at Vrije Universiteit Amsterdam in the Netherlands and lead author of the new study.

Karinen and colleagues decided to investigate whether there is any genetic basis surrounding moral views on both sex and illicit drug use. They employed a dataset from a survey of 8,118 Finnish fraternal and identical twins. Identical twins share almost all their genes while fraternal twins share roughly half of their genes. As such, twin studies are the perfect natural laboratory that allows scientists to tease out genetic factors from environmental ones when assessing behaviors.

Each participant had to answer a set of questions that measured their moral views surrounding the use of drugs and openness to non-committed sex, as well as political affiliations, religiosity, and other facts.

When comparing the results of the questionnaires between fraternal and identical twin pairs, the Dutch psychologists found that moral views concerning both recreational drugs and casual sex are approximately 50% heritable, while the other 50% can be explained by the environment in which people grew up and the unique experiences not shared by the twins. Moreover, the relationship between openness to casual sex and views on drugs is about 75% attributable to genetic effects.

“These findings run counter to the idea that within-family similarities in views toward drugs and sex reflect social transmission from parents to offspring; instead, such similarities appear to reflect shared genes,” the researchers wrote in the journal Psychological Science.

Those who frown upon casual sex and drug use (which they associate with casual sex) may be looking out for a sexual strategy that revolves around committed sex into which they’ve invested a lot of resources. People who engage in casual sex are seen as a threat to the monogamous reproductive strategy because there’s the risk of losing one’s partner in an environment where casual sex is deemed acceptable. By judging other people’s sexuality and drug use from a moral high ground, people who prefer monogamous relationships have a weapon they can wield to control the sexuality of others to serve their own interests.

“Important parts of hot-button culture-war issues flow from differences in lifestyle preferences between people, and those differences in lifestyle preferences appear to partly have a genetic basis,” Karinen added.