Confounder Of The Day: How Sexy Your Parents Were

One of the more interesting mental health results is the differing prevalence of psychiatric disorders depending on the age of the patient’s parents. This month’s JAMA Psychiatry includes a study from Denmark that conducts one of of the largest and most rigorous analyses of this effect and finds that a whole host of psychiatric diseases are more common in people born of young mothers and old fathers.

The young mother effect should be pretty straightforward. Poor women are more likely to have children at a younger age and some psychiatric diseases are more common among the poor. Women with psychiatric disorders can be more impulsive, which results in more unprotected sex and teenage pregnancy and hence children at a younger age; the children then inherit these genes and get psychiatric disorders of their own. No surprises here.

The old father effect has a potentially more interesting explanation. The male reproductive system, unlike the female reproductive system, produces gametes throughout the reproductive lifetime. Presumably older people have more time for mutations to accumulate. Some of these mutations are random, the result of passing cosmic rays or environmental toxins. Others are theorized to be a result of sex cells undergoing “selfish spermatogonal selection”, a hypothesized sorta-cancer-like process in which some sperm progenitor cells develop “selfish” mutations that increase their relative prevalence in the testes at the cost of the quality of sperm they produce – greater paternal age provides more time for this process to happen. The children of older fathers therefore end up with a higher mutational load and more likelihood of mental disorders – not to mention a host of other issues like lower IQ, less physical ability, decreased health, et cetera. It’s very plausible and according to the Danish study the child of a 45 year old father is 1.5x more likely to develop schizophrenia and 1.8x more likely to develop autism compared to the child of a 25 year old father – not a subtle effect!

But the JAMA Psychiatry study references another study, this one really clever, that casts doubt upon this finding. Petersen et al, also from Denmark (which keeps really good track of its mental health and so hosts a disproportionate number of epidemiological psych studies) proposes a much more prosaic explanation.

They find that the mental health of a child depends less on how old her father was when she was born than on how old her father was when he had his first child. In fact, after adjusting for the former effect, the latter completely disappears! For example, if Bob had Child 1 at age 20 and Child 2 at age 40, and Dan had Child 1 at age 39 and Child 2 at age 40, Dan’s second child is at higher risk for mental disorders than Bob’s, even though they both have fathers of the same age – and Bob’s younger child is at no higher risk of disorders than his eldest.

Petersen et al don’t do a great job of coming out and saying it, but I think they hypothesize that the impairments associated with mental disorders – or just the poorly functioning genes that put someone at risk for mental disorders – make it harder for a man to find a partner and start having kids. To be crass about it, if a man has dysfunctional tendencies his value on the marriage market goes down and he’s got to wait longer before he finds people willing to have children with him.

This is sad on a general level. It suggests that guys suffering from mental disorders are unlucky in love in the same way they’re probably unlucky in a lot of other things. And it means society loses what could have been a useful piece of advice for cutting psychiatric disease.

But it’s nice and liberating on a personal level. Here I am at thirty years old, unmarried, childless, and there is no way I am going to try to have kids for the next four years while I’m doing my residency. There are a couple of people in my class right now having babies – women, in fact! – and the idea of doing two of the most stressful and time-consuming things you can go through simultaneously makes my head spin. Now I get to feel a little less guilty about waiting.