Testicular size a parental predictor?

Men with smaller testicles tend to be more involved as fathers, a new study suggests. It's not yet known if ball size could be used to pre-emptively predict parenting skills though.

Published this week, in the rather aptly named journal PNAS, James Rilling and colleagues at Emory University, America, measured the volume of 55 fathers' testicles using MRI scans.

The researchers then compared testicular size with the dads' behaviour, firstly by asking the men questions to find out how involved they are as fathers, such as do you read your child a bedtime story? Do you bath them, or pick them up from school? Would you like to do this if you had the time?

As this could be viewed as a little subjective and reliant on self reporting the researchers also looked at the mothers' thoughts on the fathers' paternal skills, and they looked at the fathers' brain activity, using fMRI to scan the dads' brains as they looked at photos of their children.

The result? Men with smaller testicles got the best parenting scores in the questionnaires AND showed more activity in brain regions associated with empathy and motivation to care. Researchers ruled out the amount of the hormone testosterone as being linked to paternal involvement. More likely, they suggest it's a trade of energy expenditure between mating strategy and parenting. So, maybe men with bigger testes invest more energy in producing higher volumes of sperm, and sperm of higher quality rather than parenting?

Author James Rilling is cautious saying,  

"We're assuming that testis size drives how involved the fathers are, but it could be that when men become more involved as caregivers, their testes shrink."

So, although it's possibly tempting to look at a boyfriend's big balls and infer his future parenting skills will be lacking - this may not be the case. Since most fathers choose how involved they are in their child's upbringing. Author Rilling states:

"It might be more challenging for some men to do these kinds of caregiving activities, but that by no means excuses them,"