Being an older father isn't all bad – the first benefits associated with having an older dad have been discovered. Boys with dads who were getting on a bit at conception tended to have higher IQ and were less worried about fitting in at school than their male friends with more youthful dads.
To measure the influences of father's age on these traits, scientists first had to devise a 'geek index'. The index measured non-verbal IQ, dedication to a particular subject of interest (be it dinosaurs or Lord of the Rings) and how concerned they were to 'fit in' with other people their age.
The geekier boys may not have been the most popular at school, but they scored more highly in exams. They also had the traits that would typically set them on the path to higher earnings in the adult world.
With every year older men got before having their child, boys gained 1.5 on the geek index. To put that in context, the average geek index across the whole sample of 10,000 children was about 40.
"It is a subtle difference, but a significant one," study author Magdalena Janecka of Kings College London told IBTimes UK.
Janecka made sure that the increased geekiness they were picking up was definitely due to the fathers' age. They accounted for factors known to influence traits such as IQ, including parents' socioeconomic status. They also accounted or environmental factors to an extent, by looking at twins.
Identical twins share pretty much 100% of each other's genes, whereas non-identical twins share about half. Identical twins had more similar scores on the geek index than non-identical twins, suggesting that geekiness did have a strong genetic influence.
Link to autism risk
It's no coincidence that older fathers tend to have geekier sons, and they also have a higher risk of having autistic sons, said Janecka.
"Being a geek and having autism are very different things, but certain parts of the geekiness are also present in autism," she said.
Autism is sometimes associated with being incredibly bright – and many people with autism do have exception intellectual or musical skills, for example. But it's also a debilitating condition for many people who receive a diagnosis.
Some of the signal that the study picked up on is thought to have the same underlying mechanism as autism, as the traits can be shared and both geekiness and autism are associated having an older father.
The geekiness trait was only significant among the sons of older fathers. Daughters of older fathers were no geekier than the rest. This could be for two main reasons, Janecka said.
"Maybe our measure of geekiness was somewhat gendered and captured geekiness in boys but not girls," she said.
This is also a wider problem in autism research. Many more boys are diagnosed with the condition than girls, but that's not necessarily because more boys have autism.
"Many researchers say we cannot detect autism in females. In girls it manifests differently, but we don't have tools to capture that."
The second possibility is that girls somehow have more protection from autism than boys do.
"It could be that there's something biologically that makes females less vulnerable to whatever causes autism in boys. The same thing could be at play here with geekiness."
The research is published in the journal Translational Psychiatry.