How Your Season of Birth Is Etched in Your Brain

The season we’re born in can have far-reaching consequences. For instance, Spring babies are more likely than others to develop schizophrenia later in life, whereas Summer babies tend to grow up to be more sensation seeking. There are many more of these so-called season of birth effects. Scientists aren’t sure, but they think such patterns could be due (among other things) to mothers’ and infants’ exposure to viruses over the Winter period, or to the amount of daylight they’re exposed to, either or both of which could influence genetic expression during early development. Now Spiro Pantazatos, a neuroscientist at Columbia University Medical Center, has studied links between season of birth and brain structure in healthy adults. He thinks the association between season of birth and psychiatric and behavioural outcomes later in life could be mediated by genetic factors that affect the growth of the brain.
Pantazatos has analysed MRI brain scans taken from 550 healthy men and women at hospitals in London, England. In one analysis he looked to see if there were any particular areas of the brain that differed between people according to the season they were born in. He defined the seasons as follows: Winter (Dec 23 to March 19); Spring (March 22 to June 19); Summer (June 22 to September 21); and Fall (September 24 to December 20). For the men only, he found that those born in the Fall and Winter tended to have more grey matter in a region known as the left superior temporal sulcus (STG), as compared with men born in Spring and Summer. Looking month by month, men born at the end of December tended to have the most grey matter in this region; men born at the end of June tended to have the least.
It’s interesting that Pantazatos found a specific link with this brain region. The amount of grey matter in the superior temporal sulcus – a region that includes the auditory cortex – has previously been linked with schizophrenia, with patients tending to have reduced volume in this area.

“The current results suggest that developmental gene x environment interactions, possibly via perinatal photoperiod effects on circadian clock genes in the suprachiasmatic nuclei and elsewhere, influence developmental patterning of the STG, ultimately resulting in gross morphological differences of this region,” says Pantazatos.

The association between STG volume and season of birth may not apply to men only. Making multiple statistical comparisons across the brain carries the risk of discovering associations by chance. Pantazatos was careful to control for this risk. When he used more liberal statistical tests, he also found an association between grey matter volume in superior temporal sulcus in women, but this time the seasonal effect was reversed. Women born in the Summer had more grey matter than those born in Winter. This actually fits with other research showing that season of birth effects can be different for men and women. For example, female Winter babies tend to be less sensation seeking as adults, whereas male Winter babies grow into adults with a penchant for risk.

Pantazatos also performed another kind of analysis. He looked to see if it were possible to predict which season a person was born in, purely from looking at differences in grey matter volume across their brains. This time he found a significant result for women but not men. An algorithm picking up differences across a swathe of brain regions in the frontal cortex, parietal lobe, and cerebellum, was able to categorise a woman’s season of birth with 35 per cent accuracy. Not great, but more than you’d expect based on a guess. The null result here for men could be due to lack of statistical power. One limitation of the study is that there was no data on the participants’ location at birth. Season of birth effects related to daylight are reversed in the Southern Hemisphere and it’s possible that there were a disproportionate number of immigrant participants in some of the seasonal groups. If so, this could have diluted the signal in the analysis (this means the observed effects between season of birth and brain structure are likely underestimates).

These are intriguing results and a next step to is to carry out a similar analysis with large samples of patients. Pantazatos’ prediction is that season of birth effects related to disorders like schizophrenia will be mediated by differences in grey matter volume in relevant parts of the brain. He calls these brain differences an “intermediate phenotype” – an observable half-way house between genetic vulnerability and behavioural outcomes. “These results imply that environmental variables associated with season of birth impact human brain development,” says Pantazatos, “which subsequently exerts influences on brain structure that persist through adulthood.” Ultimately, this research helps contribute to our understanding of how environmental and genetic risk factors influence development and lead towards psychiatric disorders later in life.
Source: Wired
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