Girls are weird

Sorry for the Bart Simpson-like analysis in the heading, but this does seem really odd to me:

Girls who do not live with both parents from birth to age two may be at higher risk of starting puberty at a younger age than girls living with both parents, research published in the open access journal BMC Pediatrics suggests. The authors suggest that their findings support the hypothesis that stress in early life may influence puberty onset. The risk of early puberty onset could potentially be mitigated by interventions aiming to improve child wellbeing, according to the authors.  

A team of researchers from Kaiser Permanente Northern California Division of Research, U.S., found that girls who did not live with both parents from birth to age two were 38% more likely to begin their period before the age of 12 compared with girls who lived with both parents. Girls who did not live with both parents between the ages of two and six were 18% more likely than girls whose parents lived together to begin their period before the age of 12.

I am surprised that stress at such a young age can have such a specific biological effect 9 or 10 years later.