This was originally posted on The Whole Sky.

Schizophrenia affects about 1% of the population. It usually develops during or after adolescence and involves losing your grip on reality in a way that people find really debilitating and unpleasant.

I started researching this because I knew some people (like those with a family history) have elevated risk, meaning above 1%. If some people’s risk is higher than 1%, and the population averages 1%, people without risk factors must have lower than 1% risk. My plan was to reassure myself that my kids were unlikely to get schizophrenia, but it turns out reading a lot about schizophrenia was not a good way to put myself at ease.

If you find this topic interesting and not nervous-making, go ahead. If not, I really didn’t find anything very actionable, so feel free to stop reading.

Schizophrenia.com has a very complete list of factors that may increase schizophrenia risk. I started reading these with the intent of actually doing things differently to reduce my kids’ risk, but a lot of the suggestions either seem spurious or things you were going to do anyway (“try not to have traumatic experiences,” “don’t drink during pregnancy.”) Some of the recommendations were based on pretty ridiculous evidence, like the ones on dental x-rays and dry cleaning.

Here are the ones I found interesting.

Radiation

The radiation thing hinges on two studies. One is that people whose mothers got dental x-rays during pregnancy have higher schizophrenia risk. But the level of radiation you get from dental x-rays is really low, less than you get in a normal day from things like sunshine. I think the actual explanation is that they won’t give you x-rays if you tell them you’re pregnant, so the only people who get dental x-rays during pregnancy are women who don’t know they’re pregnant (and are probably still drinking or doing other things that aren’t good for fetuses).

Second, there’s a study that 8 rhesus monkeys exposed to high amounts of radiation in utero developed cognitive symptoms including hallucinations once they hit adolescence (not sure how you tell if a monkey is hallucinating). This made me worried about flying until I realized the low dose in this study was roughly equivalent to flying across the US 2000 times in ten days. That helped me decide I was okay with a few flights.

Rh incompatibility

Speaking of rhesus monkeys, Rh incompatibility increases schizophrenia risk, but the study came out in the year before Rhogam, so these were untreated pregnancies. Fun fact about pregnancy: if you have an Rh-negative blood type and your baby is Rh-positive, if you’re exposed to their blood (often during birth), you essentially develop an allergy to your baby. The first baby is usually okay, but after you develop the reaction your blood recognizes and attacks the blood of subsequent Rh-incompatible babies. This is one explanation for why Henry VIII had live births from his wives’ first pregnancies but so many miscarriages thereafter—he may have had an Rh-positive blood type and the wives had Rh-negative blood.

Fortunately, Rhogam (Rho(D) immune globulin) is one of the miracles of modern medicine and means that Rh-negative women like me can have healthy pregnancies, because it prevents us from developing the reaction to your baby’s blood. I assume that if you get the treatment in time, your child’s schizophrenia risk is not elevated.

Birth interval

This part is just weird (and maybe wrong). For a second child born a short time after their older sibling, the risk is fairly low. The risk increases if you’re spaced 15-26 months apart (with 18-20 months being the worst). Then risk decreases again with longer intervals, being the lowest after 45 months of space between siblings. They think it might be related to folate—pregnancy and breastfeeding deplete your folate, so your second child might not get enough if your body is still recovering from the first pregnancy. That explains why a long interval is best, but why is a short interval better than a medium one? You can only have Irish twins if you’re not breastfeeding much (since breastfeeding suppresses the return of ovulation). So perhaps mothers who don’t breastfeed are able to replenish their folate faster than mothers who do.

But even the worst-off second children have about the same rates as only children. So the folate thing doesn’t make a lot of sense. Unless first-time mothers aren’t taking prenatal vitamins at the time of conception, but start taking them and are still taking them at the time they conceive their second child, since you’re supposed to take them after birth and the whole time you’re lactating? But the study started with children born 1950-1983, and for at least part of that period I don’t think folic acid supplements were a thing.

Rather than folate, it might also have to do with an older sibling bringing home germs from daycare, or how stressed your mother is about being pregnant while minding a toddler. This all seems inexplicable enough that I decided not to take it too seriously.

Paternal age

Older fathers are more likely to have schizophrenic children. This seems like one of those that would be nice to plan around if you can, but I don’t know who really makes major life decisions based on a slight shift in likelihood of something that’s already unlikely.

Birth season

Children born in late winter in un-sunny places have higher schizophrenia risk. Some people think taking extra vitamin D is a good idea. But these are big population studies that don’t control for things like maternal infection (the other obvious difference between winter and summer babies), because checking the birthday of people with schizophrenia is much easier than finding out whether their mothers got the flu during third trimester.

(Also dredged up during this part: lesbians and baseball players are more likely to be born at the end of summer; gay men are more likely to be born six months opposite.)

Dry cleaning

Schizophrenia.com recommends recommends avoiding dry cleaning chemicals during pregnancy, but this turns out to be based on 4 people with schizophrenia whose parents were professional dry cleaners. And the Last Psychiatrist points out that 3 of these 4 dry-cleaner parents were fathers. It’s plausible that their sperm were affected, or that they contaminated their homes and wives enough to have an effect. But this doesn’t seem like a very strong basis for advice to avoid dry cleaned clothes during pregnancy. More like “avoid having babies with professional dry cleaners.” Or perhaps they should be advising men, not pregnant women, to avoid the chemicals before conception.

Marijuana / street drugs

Schizophrenia.com is very anti-drug, unsurprisingly. Slate Star Codex addresses the topic in a post on marijuana. (It’s a long post; search for “psychosis.”) There are lots of studies that show a correlation between using drugs like marijuana, LSD, and ecstasy and later being diagnosed with a psychotic disorder like schizophrenia. But it’s not clear whether marijuana and other drugs like LSD, meth, and ecstasy actually increase risk, or if teens in the early stages of developing psychosis are more drawn to drug use. I plan to tell my kids that we don’t know which way the causality goes here, and the risk is something they should consider.

There also seem to be certain genetic mutations that make this effect, if there is one, more likely for some people.

Painkillers

A British study found that women who used painkillers during the second trimester of pregnancy were several times more likely to have children with schizophrenia (and yes, they controlled for viral infections, which could be a cause both of taking painkillers and of damage to the fetus). This study looks weird to me, though, because fewer than 2% of participants reported taking painkillers during any given trimester, which seems awfully low given that about 20% of people use over-the-counter painkillers in a given week.

Other stuff

Pretty much everything else on the list can be described as good prenatal, physical, and mental health. Everything you’re supposed to do during pregnancy (eat vegetables and omega-3s, exercise, take your vitamins, try not to get sick, avoid alcohol, avoid toxins) helps reduce the child’s risk of schizophrenia. After birth, a nurturing, low-stress environment (caring for your own mental health, breastfeeding, giving the baby plenty of touch, living in less urban environments, not yelling, helping children handle stress in healthy ways) also helps. Other bad things during childhood: head injuries, emigration, psychological trauma.

studies articles