What the Freak?
New research from the US has suggested a link between the size of a man's testicles and how big a role he plays in bringing up his children.
The study, by researchers at Emory University in Atlanta, said that men with smaller testicles were more likely to take care of their baby, performing chores such as diaper changes, feeding and bath time.
Published in the Proceedings of the National Academy of Science, the study measured the testicle size of 70 men with children aged between one and two.
The men that took part in the study were aged between 21 and 55 and lived with the biological mothers of their children. Most of them were married.
The study also included taking brain scans of the men while they were shown pictures of their children to measure any nurturing activity in the brain.
The results showed that the men with smaller testicles had a greater response in the reward area of the brain that men with larger testicles.
Interviews with the men and their families also showed men with smaller testicles took a more active role in parenting duties.
The research was attempting to discover a link between promiscuity and testicle size, which are strongly linked in animals. Animals with larger testicles tend to mate with a greater number of partners, tending to put greater emphasis on creating rather than raising offspring.
The researchers said they believed the size of the testicles, through the hormone testosterone affects behavior. However, it is not clear if the process of becoming a parent has an effect on the father.