While both men and women cheat, there are important sex differences when it comes to cheating. The sex differences that influence cheating are based on two basic biological differences between men and women.

First, men and women differ when it comes to eggs and sperm. Men can produce hundreds of millions of sperm per day. By comparison, women are born with a million or so eggs, but only a fraction, roughly one egg released every 28 days over a short period of time - from puberty to menopause - has the potential to create life. Simply stated, women have about 400 viable eggs to use (and taking into account gestation, only about 20 actual opportunities to reproduce), while men are capable of producing billions and billions of sperm.

The second basic biological difference deals with gestation. Embryos grow and develop in women, not men. For men, reproduction can literally take just a few minutes of effort; while for women it involves, at the very least, a nine-month process.

From a biological point of view, men can constantly and quickly engage in reproduction while women are much more limited in their ability to do so (see, why women cheat).

These biological differences influenced our psychological desires before the invention of modern forms of birth control and still influence our unconscious sexual desires today. Men are more likely than women to think about sex and fantasize about having sex with multiple partners. In fact, a multi-billion dollar industry - pornography - exploits this basic sexual difference.

Given this basic biological difference, here are some key differences between men and women when it comes to cheating:

•Men are more likely than women to cheat with someone who is less attractive than their current partner. Women cheat up while men are more opportunistic when it comes to cheating.

•Men are more likely than women to have a one-night stand. Women are more prone to having emotional affairs.

•Men are less likely to consider leaving their partners after cheating. When women cheat, it tends to be more emotionally involved so they are more likely to consider ending their current relationship.

•Men are more likely than women to repeatedly cheat on a spouse or partner (see, for example, my husband constantly cheats).