[url=http://www.cbc.ca/news/technology/story/2013/09/10/science-testicle-size-father-parenting.html]Great dads have smaller testicles, study suggests. Brain scans suggest testicle size inversely linked to motivation to nurture.[/url]
So, the study suggests that "nurturing dads have small balls" and also implies the inverse that men with large 'nads are more likely to shirk when it comes to child-rearing duties.
Ah, testosterone. You do so much and give men something to blame when we're pricks.
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This picture is relevant.