Father knows best, but his kids might fall a bit short if he conceives them after age 50. Children of older fathers lag somewhat behind children fathered by younger men on a battery of intellectual tests, at least until age 7, according to a reanalysis of data from a large U.S. study.

This cognitive disadvantage occurred regardless of mothers’ ages, says a team led by psychiatrist John McGrath of the University of Queensland in Brisbane, Australia. Still, children of older fathers generally scored well within the normal range on the intelligence tests.

In contrast, children with older mothers scored higher than those with younger mothers on cognitive tests, regardless of fathers’ ages, the researchers report online March 9 in PLoS MEDICINE.

Increasing numbers of men are delaying fatherhood in Western nations, so scientists urgently need to explain why a link exists between fathers’ advanced ages and relatively lower cognitive scores by their children, McGrath says. He can’t yet say if or when children of older fathers catch up intellectually with children of younger fathers.

“With respect to childhood intelligence, a vast array of factors is far more powerful than paternal age,” McGrath cautions. These factors include nutrition, health care and family income.

After adjusting for parental mental health, family income and other socioeconomic factors, the link between paternal and maternal ages and children’s cognitive scores lessened but remained. The average IQ advantage for children of fathers who were age 20 when the child was born versus children of fathers who were age 50 decreased from six points to three points after these adjustments. This difference was a statistically significant, but is modest in practical terms.

Earlier studies have linked advanced paternal age to miscarriages, birth deformities, some cancers, autism and schizophrenia.

Genetic mutations that accumulate in sperm over a man’s lifetime may get passed on to children and contribute to these problems, as well as to early cognitive deficits, McGrath suggests.

Other studies suggest older fathers do not interact as well with their children, says psychiatrist Mary Cannon of Dublin’s Royal College of Surgeons in Ireland. This factor, the children’s inheritance of harmful DNA mutations from older fathers or an interaction of the two might undermine intelligence, she remarks. .

McGrath and his colleagues reanalyzed data from a U.S. study of 33,437 children born between 1959 and 1965. Each child was assessed at 8 months, 4 years and 7 years of age on tests of memory, learning, reasoning and coordination. Reading, spelling and arithmetic skills were tested at age 7.

To strengthen the findings, McGrath’s team needs to account for the tendency of first-born children to score higher on cognitive tests than later-born children, comments psychiatrist Dolores Malaspina of New York University Langone Medical Center.

Had this phenomenon swayed the results, McGrath contends, it would not have yielded opposite cognitive trends for the children of older fathers versus older mothers.

In a 2005 study, Malaspina and her colleagues found that — much as in the new study — Israeli teens who had been conceived by men over age 40 scored somewhat lower on intelligence tests than peers with younger fathers, even after accounting for the effects of birth order. A cognitive disadvantage also characterized teens who had been conceived by women over  age 40, unlike the trend found among young children with older mothers in McGrath’s investigation.

The social and environmental influences older mothers have on their kids’ intelligence may change from childhood to adolescence, McGrath suggests.

He and his colleagues did not examine, say, how children with a father who was 50 years old and a mother who was 25 years old at the time of the children's births fared intellectually compared with those whose parents were both 50 at the time of birth.

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