A study of infidelity among 64,000 Americans by David Frederick, PhD of Chapman University found that men and women differ when it comes to jealousy. Heterosexual men were much more upset than women by sexual infidelity (54% men vs 35% women) and less likely than heterosexual women to be most upset by emotional infidelity (46% men vs 65% of women). Meaning that men were much more upset imagining if their female partner had sex with another man (without being in love with him), whereas women were much more upset at the thought of their male partner falling in love with another woman (even without sex being involved). Put another way, men are more upset by sexual infidelity whereas women are more upset by emotional infidelity. A review of 16 societies found that infidelity was the most common cause of divorce. Another large study found that about 34% of men and 24% of women engage in extramarital sexual affairs. The numbers are even higher for couples who are unmarried.