Ever wonder why your summer-born sister is always so damn upbeat? Science may have an answer for that. Your genes and environment aren’t the only factors that influence your personality. Research presented at the European College of CNP Congress in Berlin has found a link between birth season and mood. In other words, the month you’re born can also affect your disposition — both in good and not-so-good ways.

“Biochemical studies have shown that the season in which you are born has an influence on certain monoamine neurotransmitters, such as dopamine and serotonin, which is detectable even in adult life,” explained Xenia Gonda, Ph.D., the study’s lead researcher and assistant professor at the department of clinical and theoretical mental health at Semmelweis University in Budapest, in a press release. “This led us to believe that birth season may have a longer-lasting effect. Our work looked at over 400 subjects and matched their birth season to personality types in later life. Basically, it seems that when you are born may increase or decrease your chance of developing certain mood disorders.”

Here’s what the researchers found:

* People born in the spring and summer have a significantly higher tendency to be excessively positive and upbeat.

* Those born in the winter were significantly less likely to have an irritable temperament than those born at other times of the year.* People born in the fall have a significantly lower tendency to depressive moods than those born in the winter.

* People who tend to have fast and frequent mood swings between feeling sad and cheerful are significantly more likely to be born in the summer, compared to those born in the winter.

“Seasons affect our mood and behavior,” said Eduard Vieta, Ph.D., head of the department of psychiatry and director of the Bipolar Disorders Program of the Hospital Clinic at the University of Barcelona, in a press release. “Even the season at our birth may influence our subsequent risk for developing certain medical conditions, including some mental disorders. What’s new from this group of researchers is the influence of season at birth and temperament. Temperaments are not disorders but biologically-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.”

So maybe it’s time you cut your happy-go-lucky sister some slack? It’s not her fault she was born in the summer.