By Michelle Roberts
BBC News health reporter, Washington DC
The animal kingdom could give important clues to rising teenage pregnancy rates, Chicago scientists say.
Animal studies offer clues to human reproduction
Stressful experiences in early life prematurely turn on female monkeys' maternal streaks, found one study.
Similarly, teenage girls who grow up without a father at home reach puberty earlier and are more drawn maternally to pictures of infants, it found.
Such factors should be considered along with more obvious socioeconomic causes, says Professor Dario Maestripieri.
The University of Chicago professor has studied the biology of reproductive and parenting behaviour in monkeys and humans.
He said there was much to be learned from animals.
His latest study, due to be published in the Proceedings of the Royal Society journal, looked at female rhesus monkeys, which are considered to be one of the animals most similar to humans.
Those females who were exposed to harsh and unpredictable maternal care in infancy showed earlier interest in infants as well as higher stress hormones during development.
Professor Maestripieri says the same is true for young girls. "Early social influences are very important for human reproduction," he told the annual meeting of the American Association for the Advancement of Science (AAAS).
"Things that go on in families in the first few years of life can have consequences way beyond childhood.
"Girls without a father at home start menstruating at an early age."
He said there were a number of theories why that might be.
"It might be the fact that there could be other males around, for example boyfriends of the mother, and the presence of an unrelated male accelerates puberty.
"It might be a genetic effect, that there is a gene in the father that makes him leave and the same gene in the daughter causes early reproduction."
But he said his work on monkeys suggested quality of maternal care was the important factor.
"Early stress can accelerate the development of maternal responsiveness in both humans and monkeys."