A study of nearly 2 million patient records in Taiwan and more than 1.5 million records from over two dozen other countries offered new insights into birth season and psychological health, finding that people born during winter months may be more prone to developing certain mental disorders, including schizophrenia, bipolar I and depression, and providing psychiatrists important clues about the mechanisms that could lead to those disorders.

The paper, published April 30 in Acta Psychiatrica Scandinavica, provides one of the most comprehensive looks at the correlation between mental disorders and birth month ever published. A team of Taiwanese researchers used data from the Taiwanese national health insurance system to determine patients' mental health diagnoses and birth dates, specifically identifying the prevalence of 34 mental disorders across that population. Overall, most of the mental disorders that were assessed showed some degree of seasonality, with the lowest risk for those born March through July and the highest for those born August through February.

In a subsequent meta-analysis, the researchers compared their results with over 1.5 million additional patients' records — gathered from 51 articles across 25 countries. That analysis indicated that winter births weren't the only ones that correlated with increased risk for certain disorders. People whose birth dates caused them to start school at a younger age faced higher rates of intellectual disability, autism and attention deficit-hyperactivity disorder compared to those born in other months. In fact, younger individuals were 13% more likely to develop either intellectual disability or ADHD than the general population, marking the highest correlations from the study. 

When researchers compared the youngest group of children with the oldest group from a particular grade, the disparity grew to an even greater extent. People from the oldest sub-section, whose birth month depended on their country of birth, had an 8% reduction in risk for intellectual disability and a 13% reduction in risk for ADHD as compared to the general population. Combined, this meant that the youngest group had a 26% greater risk of ADHD and a 21% greater risk of intellectual disability than the oldest group.

"I think that the stress of children's early life experience in school mainly causes the higher incidence of mental disorders for those with relatively young age," Chih-Wei Hsu, a researcher at Kaohsiung Chang Gung Memorial Hospital and the first author on the paper, told The Academic Times. According to Hsu, the outsized rate of some mental disorders among children who start school early could have policy implications, suggesting that governments should reevaluate their education systems to support younger students.  

The higher rates could also stem from doctors' tendencies to diagnose younger children with mental disorders due to their age rather than symptoms alone. The researchers wrote that, "The elevated [risk] in younger individuals may result from relative physiological immaturity, potentially resulting in overdiagnosis in relatively younger children and underdiagnosis in relatively older children with intellectual disability, ASD, and ADHD."

Meanwhile, people born in the winter were about 4% more likely to develop schizophrenia, and those born in autumn had a 2% higher risk of alcohol use disorder. These correlations may be related to mothers' lack of access to sunshine during pregnancy in the winter and spring, resulting in vitamin D deficiencies that can impede dopamine function in the brain. Certain types of genetic mutations associated with mental illness may also be more likely to occur in colder conditions. 

The link between schizophrenia and winter birth months had already been well-established, with research dating back to the 1990s supporting the association. But in order to further isolate the underlying factors that lead to the correlation, countries need to place a greater emphasis on data collection, Hsu said. "We need the mother's detailed data at the time of delivery, such as infection or blood vitamin level, and the occurrence of mental disorders in offspring," he added.

Viral and bacterial infections that may impinge on mental health and behavior are also more prevalent at certain times of the year, including one understudied infection called Toxoplasmosis that may be more common in the winter and spring. Developing babies may be especially vulnerable to the parasite, which can be found in around 11% of the U.S. population, although other countries may have rates exceeding 60% of the population. 

Humans often acquire toxoplasmosis through house cats, half of which are infected with the parasite. Rodents with toxoplasmosis have been shown to exhibit reckless behavior, making it more likely for them to be caught by cats and thereby spreading the disease at a faster rate. While toxoplasmosis's effect on human behavior is less clear, one study demonstrated that those infected with the parasite may be more than 2.5 times more likely to get into traffic accidents.

Not all mental disorders were connected with a particular birth month or season. The researchers found that bulimia, anorexia, dissociative disorder and post-traumatic stress disorder, for instance, were equally common across all months. The results for PTSD, at least, could be explained by its unique diagnostic criteria: It requires that a person undergo a violent or traumatic event, which can occur completely independently of one's date of birth.

To keep seasons consistent across both hemispheres, the researchers swapped southern hemisphere months so that January in the southern hemisphere, for instance, would correspond to July in the northern hemisphere. But in the future, Hsu would like to separate findings from both hemispheres to explore whether there are significant differences in prevalence between geographical regions. 

The Taiwanese team thinks that the findings could help us understand why many mental disorder diagnoses often overlap — why someone with depression may be more likely to also be diagnosed with anxiety, for instance.

"Taken together, current evidence suggests that people born in the same month have a similar biological vulnerability to a specific cluster of mental disorders or psychiatric symptoms," the authors wrote. "Future research ... should investigate clusters of disorders rather than single diseases to identify the common factors."

The study, "Month of birth and mental disorders: A population-based study and valication using global meta-analysis," published April 30 in Acta Psychiatrica Scandinavica, was authored by Chih-Wei Hsu, Chang Gung University and National Cheng Kung University; Ping-Tao Tseng, Prospect Clinic for Otorhinolaryngology and Neuroscience and National Sun Yat-sen University; Yu-Kang Tu, National Taiwan University; Pao-Yen Lin, Chi-Fa Hung, Yun-Yu Hsieh, Yao-Hsu Yang and Liang-Jen Wang, Chang Gung University; Chih-Sung Liang, National Defense Medical Center; and Hung-Yu Kao, Cheng Kung University.