Liberals Have More Sex

[C]onservatives tended to report having had secure childhoods with strong attachments to one or both parents, and low stress. Those classified as liberals reported having stressful childhoods and weaker attachments to their parents.

If food is scarce and other people are pressing in, then a liberal's willingness to challenge traditional ways of doing things, and his greater willingness to embrace outsiders (often literally; liberals tend to have more sexual encounters than conservatives) might come in handy.