By the start of the 21st century, Farrell felt he had re-examined every substantial adult male-female issue except the pay gap (i.e., that men as a group tend to earn more money than women as a group).[39] In Why Men Earn More: The Startling Truth Behind the Pay Gap—and What Women Can Do About It,[40] he documents 25 differences in men and women’s work-life choices which, he argues, account for most or all of the pay gap more accurately than did claims of widespread discrimination against women. Common to each of men’s choices was EARNING MORE MONEY, while each of women’s choices prioritized having a more-balanced life.[41] These 25 differences allowed Farrell to offer women 25 ways to higher pay—and accompany each with their possible trade-offs.[42] The trade-offs include working more hours and for more years; taking technical or more-hazardous jobs; relocating overseas or traveling overnight.[43] This led to considerable praise for Why Men Earn More as a career book for women.[44]

Some of Farrell's findings in Why Men Earn More, included his analysis of census bureau data that never-married women without children earn 13% more than their male counterparts; or that the gender pay gap is largely about married men with children who earn more due to their assuming more workplace obligations. Themes woven throughout Why Men Earn More are the importance of assessing trade-offs; that "the road to high pay is a toll road;" the "Pay Paradox" (that "pay is about the power we forfeit to get the power of pay"); and, since men earn more, and women have more balanced lives, that men have more to learn from women than women do from men.[45]