The way the authors quantified it was by taking cross cultural surveys. Basically they surveyed men and women from various cultures about their attitudes aroun

The way the authors quantified it was by taking cross cultural surveys. Basically they surveyed men and women from various cultures about their attitudes around jealousy and determined that in all cultures they studied men appeared to be more affected by jealousy. You are right that there is a rational reason why women should be jealous as well. But women have a more rational reason to forgive their mate, because it is far easier for a man to walk away from the relationship and thus the child.