Men who have smaller testes are more likely be better dads to their toddlers, according to a study published in The Proceedings of the National Academy of Sciences.

Researchers from Emory University in Georgia set out to determine why some fathers invest more energy in parenting than others, after prior research has shown that children who have a better relationship with their fathers have better social, psychological and educational outcomes.

Previous studies have shown that lower levels of testosterone in men have been linked to increased parental involvement. But the researchers were interested to see whether testicle volume, more closely linked to sperm count and quality rather than testosterone levels, also played a part.

For the study, the researchers analyzed 70 biological fathers who had a child between the ages of 1 and 2, and who were living with the child and its biological mother.

Share on Pinterest Researchers say that fathers who have smaller testes are more likely to be better dads

Both the mothers and fathers took part in interviews to determine the father’s involvement in the care of their child.

Information was gathered on whether the father changes diapers, feeds and bathes the child, whether they stay home to care for their child when it is sick, and whether they take their child for doctor visits.

After the father’s testosterone levels were measured, they underwent functional magnetic resonance imaging (fMRI), which measured their brain activity as they viewed photos of their own child with happy, sad and neutral expressions.

They were also shown photos of an unknown child and an unknown adult showing similar expressions.