Men who have smaller testes are more likely to 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.

via Dads with smaller testicles are better fathers, study shows.