A new study, published in the National Academy of Sciences PNAS, examined the relationship between testicular volume and brain activity related to nurturing in fathers.  This study built on Life History Theory, which posits that there is a trade-off between efforts associated with mating and efforts associated with parenting.  In order to test this theory, researchers took measures of reproductive biology related to mating behavior and compared them with nurturing behaviors and the brain activity related to them.  The researchers found that lower levels of testosterone and lower testes volume were correlated with higher nurturing by the father.

To examine nurturing behavior, the researchers looked at the ventral tegmental area of the brain, which is associated with the dopamine reward and motivation system.  Responses in the brain to viewing pictures of the men’s babies could be predicted by the volume of their testes and the level plasma testosterone.  According to a 2008 study in Animal Behavior, higher testosterone is associated with greater mating success.  Combined with the results of the present study, it confirms the Life History Theory claim that mating efforts are inversely related to parenting efforts in men.

A BBC News analysis of the current research identified that the men with smaller testicles also reported higher levels of involvement with feeding, bath time, and changing of diapers.  However, the reproductive biology alone did not explain the differences.  Researchers also noted that there were cultural expectations that impacted the role played or expected to be played by some of the fathers.  One of the researchers in the study emphasizes that this only means that some men are more naturally inclined to a nurturing role, but other fathers must step up and put in the effort to be strong caregivers for their children.

There is also some indication in the literature that nurturing may impact reproductive biology.  Specifically, when men become involved fathers, their testosterone levels go down.  More research still needs to be done to determine whether or not becoming a father biologically changes a man and, if so, the nature of those changes.

According to Time Health & Family, a similar effect was not seen on testicle size.  Simply having children did not change the size of testicles.  Based on research to date, it is not possible for scientists to say whether the biology is affecting the behavior, behavior affecting the biology, or if there is a much more complex relationship between these areas.