Brain charges at or around birth may give you a shy or extrovert personality even decades later, say researchers.
The bashful brain: Amygdala activation caught on camera
Experts at Harvard University claim that brain scans of 21-year-olds provide evidence that traits such as social anxiety or bashfulness may have a physical root - and one which could have its origins in the cradle.
However, their research does not prove definitively that these brain changes are the cause of the problem - and a British expert says that parents of shy children should not give up hope that they will come out of their shells.
The research, published in the journal Science, focuses on a part of the brain called the amygdala.
Other research has linked this region to the way the brain processes emotional information - such as fear or anger - from faces.
Now the Harvard team believes that it may play a crucial role in how the brain reacts to all kinds of novel objects and situations, not just people.
The message to parents with a shy child is - don't give up hope
Dr Simon Killcross, Cardiff University
They suggest that a differently developed amygdala may contribute to children who are "inhibited" - timid with new faces and situations.
Their research took children who were rated as shy in personality tests taken when they were just two years old.
Almost two decades later, they were re-tested - and their brains scanned to see if there were any differences in amygdala activity compared with other volunteers.
They found that, when presented with pictures of familiar, and unfamiliar faces, the amygdala "lit up" more strongly when the "shy" group were shown the unfamiliar.
Adults who had been more outgoing toddlers showed less activity in this situation.
Dr Carl Schwartz, who led the project, said: "We found that individual differences in temperament are associated with persistent differences in the responsivity of the amygdala, after more than 20 years of development and life experience.
"It's only by understanding these developmental risk factors that one can really intervene in the lives of children early, to prevent suffering later in life."
Dr Simon Killcross, from the School of Psychology at the University of Cardiff, said that the study did not prove a "causal" link between differences in amygdalal response and shyness early and later in life.
He said: "The message to parents with a shy child is - don't give up hope.
"What this study does suggest is that is that being a shy child is not a determinant of shyness in adulthood.
"While two out of the adults who were shy as children were diagnosed with some kind of social phobia, this means that 11 were not."