BERLIN ― The season during which an individual was born appears to influence adult temperament, which, in turn, may lead to the development of a mood disorder, new research shows.
A study of more than 300 college students in Hungary showed that the participants who were born in winter were less likely to have cyclothymic, hyperthymic, or irritable temperaments compared with their peers who were born during other seasons.
However, winter-born students were more likely to have depressive temperament than were autumn-born participants.
"These findings run parallel to results found in a small, previous study," lead author Xenia Gonda, PhD, assistant professor in the Department of Clinical and Theoretical Mental Health at Semmelweis University in Budapest, Hungary, told Medscape Medical News.
"Cyclothymia is associated with bipolar II disorder; we saw higher cyclothymic scores in those born in summer; and there's a slightly higher number of bipolar II patients born in summer. Major depressive patients are a bit more likely to be born in the winter."
However, she added that more research is now needed.
"These are small effects, so no recommendations for clinicians just yet. This is really about how our environment and our genes determine who we are," said Dr Gonda.
The study was presented here at the 27th European College of Neuropsychopharmacology (ECNP) Congress.
"Season of birth has been considered to be an important element in determining personality and illness already in the prescientific medical era, and this tradition lives on in several contemporary concepts," write the investigators.
They note that previous studies have suggested a link between season of birth and disorders such as schizophrenia and unipolar and bipolar major depression. However, how it relates to affective temperaments has not been assessed before.
Dr Gonda added that biochemical studies have shown that season of birth influences monoamine neurotransmitters, such as dopamine and serotonin, even in adulthood.
"This led us to believe that birth season may have a longer-lasting effect," she said.
"Date of birth could be related to many things: date of conception, the season spent in your mother's womb, and the first months after birth, nutrients available, how much light you get. Also, how much activity is a pregnant woman doing? Is she walking a lot or hiking if in the summer? These are all important factors. But season of birth also affects certain aspects of neurotransmitter activity."
A total of 366 university students (70% men; mean age, 20.6 years) were enrolled in the study and asked to complete the standardized Hungarian version of the Temperament Evaluation of Memphis, Pisa, Paris, and San Diego-Auto (TEMPS-A) questionnaire.
"We compiled 11 dummy variables for the month of birth from February to December and three others for the season of birth from spring to autumn; that is, we used January and winter as a point of reference," report the investigators.
Results showed that, compared with the participants born during winter, those born during summer had significantly higher scores on the TEMPS-A for cyclothymic temperament (P = .0345), and those born in spring and autumn had higher scores for hyperthymic temperament (P = .007 and .014, respectively).
In addition, those born in the autumn had a significantly lower tendency to depressive temperament compared with those born in winter (P = .014); and those born in winter were less likely to have irritable temperament than were those born at any other time of the year (P = .011, .0001, and .0003, respectively, for spring, summer, and autumn.)
"Our results are in line with clinical observations concerning the seasonal variation of onset and hospitalization due to affective episodes," the investigators note.
"This is especially important since affective temperaments are conceived as the subaffective and subclinical manifestations of major and minor affective disorders, indicating a risk for the development of these disorders and also exerting a possible patoplastic effect," they write.
Dr Gonda added that they do not yet know about the mechanisms involved.
"What we are now looking at is to see if there are genetic markers which are related to season of birth and mood disorder," she said.
"Seasons affect our mood and behavior, [and] even the season of our birth may influence our subsequent risk for developing...some mental disorders," said Eduard Vieta, MD, PhD, professor of psychiatry and director of the Bipolar Disorders Program of the Hospital Clinic at the University of Barcelona, Spain, in a release.
"What's new from this group of researchers is the influence of season at birth and temperament," he added.
Dr Vieta, who is also chair of the ECNP Communication Committee, was not involved with this research. He noted that temperaments are not disorders but rather are biology-driven behavioral and emotional trends.
"Although both genetic and environmental factors are involved in one's temperament, now we know that the season at birth plays a role too," said Dr Vieta, adding that the findings are "quite intriguing."
The study authors report no relevant financial relationships.
27th European College of Neuropsychopharmacology (ECNP) Congress. Abstract P.1.k.008. Presented October 19, 2014.
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Cite this: Birth Season May Influence Temperament, Mood Disorders - Medscape - Oct 21, 2014.