Part 1 of this series discussed the nation's first paid leave policy, implemented in California in 2004, which provides 6 weeks of paid leave to new parents. Even though the program is gender-neutral, mothers took family leave at much higher rates and for a longer period than did fathers.

However, we did see increased leave-taking in fathers when they have sons. Fathers with sons are more likely to take a longer period of family leave than fathers of daughters, and fathers of sons are also more likely to take sole family leave, while the mother returned to work. This paternal son-preference is consistent with previous research: Fathers are more likely to spend more time with sons than daughters and in the case of divorce, fathers are more likely to have custody of sons than daughters.

Additional analysis found that unmarried men who father sons are more likely to acknowledge paternity than unmarried men who father daughters. The effect was slight--a male baby is 4% more likely to have a father who acknowledges paternity, relative to a female baby--but statistically significant.

So, why do fathers take more paternity leave with sons? "First, it may be that fathers get more utility from spending time with their sons than daughters," according to a study of California's paid family leave program. ("Greater utility" is economist-speak for "we like it more." Fathers might be spending more time with their sons because they like them more, simply because they are sons.)

Another reason could be that both parents believe that it's more productive for the father to spend time with their sons. The researchers found this explanation "less plausible given that it is unlikely that fathers have a relative advantage in caring for boys around the time of birth." Since newborns of both genders require the same amount of care, any time difference is more likely due to the parent's preference rather than the child's needs.

Other researchers have found a relationship between fathers' wages and child's gender. Fathers who have sons have higher wages than fathers who have daughters. "Most notably, men' s labor supply and wage rates increase more in response to the births of sons than to the births of daughters," the study concludes, which might suggest that fathers who have sons work harder because they are planning on investing more in their sons.

It is unclear whether the decision to invest more time and resources into sons is a preference by fathers or a joint decision made by mothers and fathers together. If men indeed prefer sons over daughters, having family-friendly office policies might not be enough to change men's attitude about gender equality. Employers can't force parents take the same amount of paid leave for each of their children, although social pressure may cause some parents to adjust the amount of family leave to be closer in length to their peers' and coworkers'.

Most advocates of paid family leave hope that work-life balance programs will promote greater gender equality by supporting women in their careers, but there is direct relationship between liberal parental leave policies and higher pay gaps between men and women.

Time out of the workforce is associated with lower wages and companies that don't have a gender-neutral family leave policy usually offer longer maternity leave than paternity leave. It's natural for parents to want to spend as much time with their newborns as possible, and policies that are mom-friendly are harmful to women's careers.

This is part 2 of a 4-part series.