Can someone's morals evolve?

Can someone's morals evolve? President Obama, who endorsed gay marriage last week, has described his views on gay marriage over the past few years as "evolving." Can someone's moral principles really evolve?

Yes. While some of the growing national acceptance of gay marriage is the result of turnover -- as older, more socially conservative people die off -- there has been a significant amount of mind-changing among members of the public.

Many sociologists believe that morality derives from intuition rather than reason, and our rational explanations for moral positions are simply post hoc justifications.

One of the primary movers of our intuitive sense of morality, the argument goes, is called the cohort effect. According to this theory, people shift their views so they match those of their peers. That doesn't explain why those peers changed their views -- at some point, someone has to have changed opinions on his or her own -- but the cohort effect does serve to accelerate a broad moral shift across society.

On an individual level, moral judgments are also driven by visceral feelings of repugnance. Social conservatives, for example, tend to find the thought of homosexual activity somewhat disgusting. While it's difficult to change one's general proclivity for disgust, there is ample evidence that repeated exposure to a particular image can lessen its impact.

Medical students, for example, consider cadavers far less disgusting by the end of medical school, even though they find the smell of urine or the idea of accidentally drinking a stranger's soda as disgusting as ever.

The same scenario can also play out in societal attitudes toward homosexuality. Increased exposure may have lessened the negative responses to it, and, as the sense of revulsion fades, so does opposition to gay marriage. In this sense, evolution is a reasonable metaphor for the process by which opponents of gay marriage have adapted to a changing environment.

Vice President Joe Biden took some heat for his statement on "Meet the Press" that "Will & Grace probably did more to educate the American public [about homosexuality] than almost anything anybody's ever done so far." There is research, however, to suggest he's right about the influence of TV. Economists Robert Jensen and Emily Oster found that access to cable television significantly changed the social views of Indian villagers. Those who could watch the newest shows, with their cosmopolitan mores, became more opposed to domestic violence, less likely to prefer sons over daughters, and more likely to enroll their children in school. Cable-watchers also had fewer children, on average.

Despite this research on the roles of disgust and passive television watching, the application of reason does play into the development of individual morality. People sometimes change their mind as a result of simple rational persuasion, even if it's difficult for sociologists to study.