Diaper-changing dads have small testicles

The news reported on a new study:

Researchers analyzed male testicle size and were able to correlate it to a man's parenting strategy in a new study. The researchers findings suggest that males with larger testicles were less involved parents — in general they spent less time caring for their children and had a decreased brain response to images of their child. Having a father around has multiple benefits for children — behaviorally, socially, and health-wise. But, for some reason some fathers still choose not to invest in parenting their children. And this number of absent fathers seems to be growing. The researchers, led by James Rilling of Emory University, wanted to know why some fathers were more involved than others. "Our study is the first to investigate whether human anatomy and brain function explain this variance in parenting effort," study researcher Jennifer Mascaro, said in a press release.

It is faulty reasoning to relate this to absent fathers. The study abstract says:

Despite the well-documented benefits afforded the children of invested fathers in modern Western societies, some fathers choose not to invest in their children. Why do some men make this choice?

The full study is behind a paywall, but all they found was that the men who change a lot of diapers have smaller testicles and lower testosterone. SciAm reports:

A 2011 study in the Philippines suggested that men who have high testosterone levels are more likely to marry. Even so, those men who are eventually more involved in day-to-day child care duties — such as changing diapers, running the bath or kissing scraped boo-boos — see their testosterone levels drop more than men who remain aloof after having children. But testosterone has many roles in the male body, so it wasn't clear whether the drop in the male hormone occurred because men were investing more in parenting than in mating. ... Men with bigger testes had a more hands-off parenting style, and the reward centers of their brains activated less when the men viewed their children's pictures. These fathers also tended to have higher testosterone levels.

I can believe that changing diapers causes testosterone levels to drop. I believe that I have experience that myself, altho I did not measure the levels. This could also explain why pediatricians and other men doing child care seem to have less masculine characteristics. Where I disagree is the argument that high-testosterone dads are less "invested" in their kids. Those dads have wives to change the diapers, and the wives are happy to do it. The dads are invested in many other ways, besides changing diapers.