Be grateful for the times mom changed your wet nappies and rocked you to sleep. All of that fussing, it appears, may have made you smarter.
Experiments on rats by Canadian researchers suggest that mothers' nurturing stimulates neural connections in their babies' brains and enhances learning. Those offspring subsequently scored higher in intelligence and memory tests.
The researchers said the results, which appear in the August issue of Nature Neuroscience, are broadly applicable to humans, too.
"It's never nature versus nurture. The influences are inseparable," said Michael Meaney, a neuroendocrinologist at McGill University.
"Activity of the genes is always influenced by the environment. And the most important feature of the environment for an infant is mother."
Other researchers described the findings as impressive.
"The stimulation provided by these mothers is certainly a large part of what causes the brain to develop more extensively," said neuroendocrinologist Bruce McEwen of Rockefeller University in New York.
However, some scientists cautioned against comparing rats' and humans' mothers too closely, or attributing infants' intelligence to relatively small differences in parenting styles.
"I don't want to put any more pressure on mothers," said Rebecca Burwell, a psychologist at Brown University.
"The rat mothers showed differences in skills, but they all were in the normal range. So it doesn't really speak to parental abuse. Some individuals may be very sensitive to subtle variations in parenting."
The McGill team divided 32 female rats into two groups. Those in one group gave a high level of care to their offspring, including stroking, grooming, licking and attentive nursing.
Mothers in the other group were mostly indifferent.
When the baby rats matured, they were tested 15 times over three days in a swimming maze in which they had to find a small platform submerged in a shallow tank.
The rats scored about the same on the first day of testing. But the offspring of the attentive mothers scored higher on days two and three. The smarter rats also scored high on other tests throughout their lives, she said.
"There is evidence for a direct relationship between maternal care and hippocampal development, and spatial learning in adulthood," Meaney said.
"This is experience-dependent development: use it and it grows. Don't, and it disappears."
He said that the findings suggested proper nurturing could strongly influence how the body carried out genetic instructions. - Sapa-AP