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Do men have a biological clock of sorts? A large new study suggests they may. Compared to younger fathers, older fathers� children were found to be significantly more at risk for a host of psychiatric disorders, according to the study, published Wednesday in the journal JAMA Psychiatry. For example, the children of fathers ages 45 and over were three times more likely to have an autism spectrum disorder, 13 times more likely to have ADHD, and 24 times more likely to have bipolar disorder than the children of fathers aged 20 to 24. For the study, researchers analyzed data from 2.6 million children born in Sweden between 1973 and 2001, making it one of the largest and most comprehensive studies on the effects of paternal age. �The working hypothesis is that as men get older and their sperm continue to replicate, that there are more chances of having mutations in the base pairs of the DNA,� says Brian D�Onofrio, lead study author and associate professor in the Department of Psychological and Brain Sciences at Indiana University. �Those genetic mutations are associated with psychiatric problems.� The age-related effects were gradual, researchers found; a 35-year-old man�s child had greater risks than the child of a 25-year-old man. A 2012 study done by researchers in Iceland indicated that as many as 20% to 30% of cases of autism and schizophrenia may be linked to fathers� advanced age. Unlike findings on conditions such as Down syndrome, the Iceland study found the mother�s age made no difference. For that study, researchers looked at genomes of 78 sets of parents and offspring. While most studies on this subject simply compare children born to young fathers with those of older fathers, D�Onofrio calls that method �comparing apples and oranges, because we know that young fathers differ on many things compared to older fathers.� Instead, D�Onofrio and his colleagues compared siblings; looking at the outcomes when the same man has a child in his younger years and then again later in life. �That enabled us to get a better understanding of what�s truly due to the advancing father�s age at childbearing,� says D�Onofrio.