I'm leaning more towards the stress caused on the parents (mother specifically). Mothers tend to be more emotional and bonded with their kids due to hormones during and after pregnancy that makes it so the mother has the instinct to protect and nurture the offsprings. So I suppose they tend to worry over the children more which increases stress levels that could lead to several diseases not limited to heart issues. More children means more to worry about. I know my mother freaks out more than my dad when one of us gets sick. Gender wise, boys tend to be more of a handful than girls especially when they reach the age where they feel they need to assert themselves. Though, of course, there are exeptions to that. I don't suppose they took note of the children's age group and whether or not they're the naughty vs not? And the no kid, I was told before that women who don't give birth are prone to certain forms of cancer (not sure about that though).