Incest is how common? We know that there are good reasons not to be sexually attracted to relatives. But just how many people have sex with family?
The Longitudinal Study of Violence Against Women asked college-age men and women about sexual experiences before age 14. I counted up the number of respondents who said they had some kind of sexual contact with relatives. I divided the counts into those who did it because they wanted to and those who did it against their will.
Percent having sexual contact with a relative before age 14 (N = 2,356)
Consensual plus forced 9.5
Consensual plus forced 2.4
Notice how it's consensual most of the time. Why the gender difference? Part of it seems to be the greater number of older as opposed to same-age family partners for girls, but I'm not sure about the rest of it. Perhaps males who are attracted to family are more likely to have multiple partners. Women might be more willing to talk about these experiences, but I doubt it.
Keep in mind that while the survey doesn't specify, I'm sure that some of these cases, perhaps most, are "kissing cousins."
The numbers, I'm sure, would be higher if the survey had asked about experiences after age 13. I remember my buddy in college telling me how he had just done unspeakable things with his first cousin. This, of course, is a broad definition of incest.