I was reading an article about the book by Dr. Brizendine titled, The Male Brain. The author was asked why some men may be built to cheat. Here's her response:

In humans they have identified, so far, about 17 different lengths of [the vasopressin receptor gene]. There are several studies that have shown that those males with the longer version are more likely to be married, and their wives are more likely to say they have a happy, successful marriage and there hasn't been any infidelity. The ones with the shorter ones are more likely to be bachelors.

So it's safe for me to admonish women and say that the only part of the male anatomy we should be measuring is the length of his vasopressin receptor gene. And the size of his ego.

If you want to Read more: http://content.time.com/time/health/article/0,8599,1976274,00.html#ixzz2hK7qDHIc