Enquist et al on lactation fetishes is one of my favorite papers.

They wonder – as we've all wondered at one point or another – how people develop fetishes. One plausible hypothesis is "sexual imprinting". During childhood, you have a critical period (maybe ages 1 to 5) where you figure out what sex is. If you see some weird stuff during that time, you could end up with a fetish. For example, a child who sees latex used in a sexualized way (for example, they catch a glimpse of a sexy movie where someone is wearing latex) might grow up with a latex fetish.

Enquist et al realize lactation fetishes offer a natural test of this hypothesis. Children with younger siblings will see a lot of breastfeeding going on during their critical window; children without younger siblings will see less. Since it's easy to ask people how many siblings they have, you can see if younger siblings correlate with lactation fetishes.

They survey some online lactation fetishist communities and ask everyone how many older and younger siblings they have. Although by chance we would expect an equal number of both, in fact the fetishists have many more younger than older siblings: