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To an infant, a mother's touch is warm and comforting. Now a Canadian researcher has found out how it can also trigger profound changes beneath the skin -- how a caregiver's touch can physically alter a baby's genes. McGill University's Michael Meaney's revolutionary findings may finally end the long debate over whether heredity or the environment play the primary role in shaping an individual's personality.

The nature-nurture debate has become less fierce in recent years: Many scientists now accept that a mysterious dance between genes and environment determines who we become. Dr. Meaney and his colleagues have uncovered an important step in that dance, by showing for the first time how conditioning can change the chemical structure of the genes we inherit -- in effect, switching them on or off.

Their work is with rats. But his team has just begun a $4-million, five-year study that could be the first in the world to confirm that it also applies to humans.

There are two different kinds of rat mothers -- those that lick their pups a lot and those that don't. Dr. Meaney found that, under provocation, the high-licking mothers' offspring produced less of the stress hormone cortisol. They are more stable individuals that are not as easily panicked.

Picture half a dozen rats in a cage, eating. Dr. Meaney claps his hands loudly, and all the animals freeze. Some rats almost immediately go back to gobbling their food, realizing that the researcher doesn't pose a real threat. But others will remain immobilized for up to 10 minutes, and may never go back their lunch. The difference? The timid rats are the offspring of mothers who didn't lick them much.

How can licking make that much of a difference in personality -- is it due to their genes, or the way they were raised? The answer is both. In essence, the high-licking moms produce changes in their babies' DNA. Their pink tongues somehow flick on the same chemical switch that turns genes on and off in a developing embryo.

In the fetus, this process -- which scientists call methylation -- allows development of the brain and organs to proceed in an orderly fashion. In the baby rats, the high-licking mothers somehow switch on a gene that restricts the production of cortisol. The low-licking mothers do not, so their pups produce much higher levels of the stress hormone.

Cortisol helps prepare the body to deal with a threat -- such as the possibility that a clapping researcher means them harm. In short bursts, it can save an animal's life. But over the long term, a heightened stress response has been linked to diabetes, heart disease, mental illness and other serious ailments in both humans and lab animals.

"We have now studied that particular gene down to the level where we know what maternal care is doing to turn it on or off," says Dr. Meaney.

He sees his work as helping to reframe the nature-nurture debate, which dates back to at least the 13th century. Extremists on one side have argued that all animal behaviour is instinctive. Hard-liners on the other side have taken the position that experience alone determines behaviour. The argument flares every decade or so, most recently with the mapping of the human genome. Now, the question is no longer whether genes are more important than the environment, says Dr. Meaney. The question is how the environment physically alters genes to produce individual differences.

The genetic changes in the baby rats affected not only the stress response but also cognitive development, since high levels of cortisol inhibits the growth of the hippocampus, a part of the brain involved in memory and learning. Cortisol may also influence how well young animals pay attention. Alison Fleming, a researcher at the University of Toronto, has found that rats brought up without mothers -- and that don't have attentive researchers stroking them with paint brushes to stimulate licking -- grow up with more attention problems than animals that get that physical stimulation.

Could the same things be true in humans? We don't regularly lick our babies or stroke them with paint brushes, and Dr. Meaney isn't suggesting that anyone start. His hypothesis is that tactile contact has the same effect. "Mothers don't lick their babies, but they hold them. And they probably tickle them and touch them and stroke them and rub their hair."

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