Our unacceptably high breast cancer rates and the "ideal" of baby sleeping alone are recent products of western society, Durham University anthropologists have claimed.
Looking at other societies leads us to question conventional medical wisdom applied in our own, they argued.
Levels of the reproductive hormone oestrogen are abnormally high in western women, and probably account for the high rates of breast cancer in richer countries, Dr Tessa Pollard said yesterday.
And parent-infant sleep contact is a human universal - except in richest societies, said Dr Helen Ball.
They were speaking at the British Association's Festival of Science at Salford, Greater Manchester yesterday.
"Women in rich countries are experiencing lifetime exposure to their own oestrogen at levels much higher than those to which our species is adapted," said Dr Pollard.
Oestrogen is produced by the ovaries during menstrual cycles. Western women have more menstrual cycles than women living in subsistence societies, she said.
This is because they mature earlier, have menopause later and spend far less time breast feeding.
"And the less food you eat and the more physically active you are, the less oestrogen your ovaries produce," she added.
"Oestrogen is the main risk factor for breast cancer, so it's a health problem," she said. However, oestrogen protects from heart disease and is one of the reasons that women suffer from less heart disease than men, she said.
Dr Pollard is currently studying the oestrogen levels in immigrant and UK-born Pakistani women. Preliminary results from a related study shows that women born and raised in the UK have higher levels of oestrogen than those born in Asia, she said.
Dr Ball thinks it is time to look at the conventional wisdom that healthy infant sleep is best achieved alone in a cot. "We never ask if it's safe to sleep alone, and get so hung up about sleeping with babies," she said.
Worldwide cot-death rates have fallen dramatically now that infants sleep on their backs. But while much has been written about the potential dangers of parents and infants sleeping together, large scale studies have shown that it is risky only if the parents are smokers, she said.
Parent-infant bed sharing may be much more common than thought. In a study of 253 families, Dr Ball found that about half of infants slept with their parents at least occasionally, and even by three months of age, one in four were regularly sleeping with their parents. "There is no evidence that normal bed sharing is dangerous," she said.
Accidental asphyxia can occur, but this is associated with drug or alcohol excess, she said. "If it's not a problem for parents, then its fine. It should be an individual decision on the part of each parent, but all parents ought to be provided with information. The vast majority of parents who don't intend to, end up doing it at least sometimes," she said.
Safe practices include making sure there are no gaps around the furniture, keeping pillows away from the infant, not having too many bedcovers and making sure that everyone in the bed knows where the infant is, she said.
The position that breastfeeding mothers adopt most frequently when they are sleeping with their infant is also the safest. The mother lies on her side with her knees drawn up under the infant and her upper arm lies above the infant's head, she said.
Bed-sharing may be beneficial because of its effects on breast feeding, said Dr Ball. It makes breast feeding at night easier, and if mothers are nursing round the clock, it stimulates milk production. Both help women carry on breast feeding for longer, she said.
The most common reason given for bed-sharing was ease of breast feeding. Parents also did it to settle babies and if they had no where else to put the infants, she said.