The unsatisfying answer is that moral foundations theory doesn’t explain why the foundations get applied in the ways that they do—that differs between cultures and involves a lot of path-dependence through history. But Haidt’s theory does at least provide some guidance for conjecture, so I’ll speculate about why sexual liberty became important to liberals based on the liberty/oppression foundation.
The way that Haidt describes it, for American liberals the liberty foundation is primarily about wanting to protect sympathetic victims from oppression. Telling the story from that point of view, sexual restrictions have been a form of oppression, involving shunning and other social punishments for victims like women who had sex outside of marriage or men who loved other men. With the sexual revolution, liberals threw off these arbitrary and oppressive restrictions, and brought us much closer to a world where no one can stop you from being yourself (holding your sexual identity openly without fear of reprisal) or from having sex with who you want to, how you want to (as long as you are consenting adults).
Government regulation in many other aspects of people’s lives has not involved such obvious oppression of sympathetic victims.
Now, could someone return the favor and offer their speculation about how the purity foundation led liberals to value sexual liberty? Vladimir_M mentions that people tend to apply purity-based morality to sex, which is true, but they tend to apply purity at the object level as a reason to restrict sexual activities. Sexual liberty would be applying purity at the meta level as a reason to allow sexual activities. Sex can spread disease (the purity/contamination framework originally evolved for avoiding illness), involves the body in a way that is closely related to many elicitors of disgust, and because of its evolutionary importance people are prone to having strong intuitions & emotions about sexual activities that don’t readily fit in other foundations. So it’s understandable that people would tend to get squeamish about various sexual practices and want to restrict them based on purity concerns. It’s not clear to me how those purity concerns jump to the meta level and reverse direction.
The unsatisfying answer is that moral foundations theory doesn’t explain why the foundations get applied in the ways that they do—that differs between cultures and involves a lot of path-dependence through history. But Haidt’s theory does at least provide some guidance for conjecture, so I’ll speculate about why sexual liberty became important to liberals based on the liberty/oppression foundation.
The way that Haidt describes it, for American liberals the liberty foundation is primarily about wanting to protect sympathetic victims from oppression. Telling the story from that point of view, sexual restrictions have been a form of oppression, involving shunning and other social punishments for victims like women who had sex outside of marriage or men who loved other men. With the sexual revolution, liberals threw off these arbitrary and oppressive restrictions, and brought us much closer to a world where no one can stop you from being yourself (holding your sexual identity openly without fear of reprisal) or from having sex with who you want to, how you want to (as long as you are consenting adults).
Government regulation in many other aspects of people’s lives has not involved such obvious oppression of sympathetic victims.
Now, could someone return the favor and offer their speculation about how the purity foundation led liberals to value sexual liberty? Vladimir_M mentions that people tend to apply purity-based morality to sex, which is true, but they tend to apply purity at the object level as a reason to restrict sexual activities. Sexual liberty would be applying purity at the meta level as a reason to allow sexual activities. Sex can spread disease (the purity/contamination framework originally evolved for avoiding illness), involves the body in a way that is closely related to many elicitors of disgust, and because of its evolutionary importance people are prone to having strong intuitions & emotions about sexual activities that don’t readily fit in other foundations. So it’s understandable that people would tend to get squeamish about various sexual practices and want to restrict them based on purity concerns. It’s not clear to me how those purity concerns jump to the meta level and reverse direction.