They are, according to new research.
Scientists studied 400 people and matched their personality type to the season when they were born to test their theory that people born at particular times of the year have a greater chance of developing certain personality traits.
This, they believe, is because the seasons have an affect on certain chemical substances in the brain, such as dopamine and serotonin, which control mood.
They then discovered that babies born in the summer were much more likely to suffer mood swings when they grow up.
In contrast, those born in spring tended to be excessively positive, upbeat and optimistic.
They also found that those born in the autumn were less likely to be depressive, while winter babies were less likely to be irritable.
The study, conducted by scientists in Budapest. Lead researcher Professor Xenia Gonda told the Sunday Telegraph: "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.
"We are now looking at is to see if there are genetic markers which are related to season of birth and mood disorder."
The study was presented at the annual conference of the European College of Neuropsychopharmacology (ECNP) in Berlin, Germany.
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