Babies born in the summer are much more likely to suffer from mood swings when they grow up, while those born in the winter are less likely to become irritable adults, scientists claim.
Researchers studied 400 people and matched their personality type to the time of year when they were born.
They claim that people born at certain times of the year have a far greater chance of developing certain types of temperaments.
The scientists said this was because the seasons had an influence on certain chemical substances in the brain, such as dopamine and serotonin, which control mood.
They discovered that "cyclothymic" temperaments, characterised by rapid, frequent swings between sad and cheerful moods, were significantly more common among people born in the summer.
A "hyperthymic" temperament, with a tendency to be excessively positive, was significantly more common among those born in the spring and summer. The experts also found that those born in the autumn were less likely to be depressive, while those born in winter were less likely to be irritable.
The study may provide a clue as to why some of the nation's best known personalities are good-natured, while others are slightly grumpier.
The scientists, based in Budapest, said that more research was needed to find out why the seasons affected brain chemistry.
Assistant Prof Xenia Gonda, the lead researcher, said: "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. 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."
Prof Gonda added: "We can't yet say anything about the mechanisms involved. What we are now looking at is to see if there are genetic markers related to season of birth and mood disorder."
The study is being presented today at the annual conference of the European College of Neuropsychopharmacology in Berlin.
Prof Eduard Vieta, of the college, said: "Although both genetic and environmental factors are involved in one's temperament, now we know that the season at birth plays a role, too.
"And the finding of 'high mood' tendency (hyperthymic temperament) for those born in summer is quite intriguing."