New research shows children born to older moms are likely to be smarter than those born to younger mothers, and a study published in the International Journal of Epidemiology found the new research is a dramatic turnaround from 40 years ago.

The shift is due to changes in the characteristics of women who wait to have children, say the researchers from the London School of Economics and Political Science, and the Max Planck Institute for Demographic Research.

Today's older mothers tend to be well educated, are less likely to smoke, and are established in professional occupations, characteristics older moms in the past didn't necessarily possess.

In addition, more women are having their first child at an older age, and first-born children usually perform better on cognitive ability tests, perhaps because they get more attention from their parents that siblings born after them.

In the past, older moms were giving birth to their third or fourth child.

"Our research is the first to look at how the cognitive abilities of children born to older mothers have changed over time and what might be responsible for this shift," said researcher Dr. Alice Goisis.

Researchers analyzed data from three British studies — the 1958 National Child Development Study, the 1970 British Cohort Study, and the 2001 Millennium Cohort Study. Children's cognitive ability was tested when they were 10 or 11 years old.

In the 1958 and 1970 studies, children born to mothers aged 25-29 scored higher than children born to mothers aged 35-39. In the 2001 study, this result was reversed.

When the researchers took the mothers' social and economic characteristics into account, the differences across the studies disappeared. This indicates that the changing characteristics of women who have children at an older age were probably the reason for the differences.

"It's essential to better understand how these children are doing given that, since the 1980s, there has been a significant increase in the average age of women having their first child in industrialized countries," Goisis said.

"Cognitive ability is important in and of itself but also because it is a strong predictor of how children fare in later life — in terms of their educational attainment, their occupation and their health."

The average age when a mother has her first child is on the increase in the United States. In the last 45 years, the mean age of women giving birth for the first time increased by five years. And according to the National Center for Health Statistics, the number of women giving birth after the age of 40 almost quadrupled between 1980 and 2004.