Parents often say their child’s personality was apparent from day one, but can adult personality really be predicted  from the way a baby behaves while  still in diapers?

That was the question researchers investigated in a study published Tuesday in the journal Molecular Psychiatry.

 Researchers followed 135 children  from infanthood through early adulthood, and found that, for boys at least, being a fussy, reactive infant in the first few months of life was associated with a stronger neurological reaction to unfamiliar faces at age 18 — a reaction researchers believe signals a propensity toward social anxiety and possibly depression later on.

When the study subjects were 4 months old, researchers evaluated whether they were “high reactive,” meaning they fussed and cried when presented with loud noises or unfamiliar smells, or “low reactive,” meaning they didn’t react in this fearful or agitated manner when presented with new stimuli. Researchers suspected that the “high reactive” infants would continue to have a negative response to unfamiliar stimuli up through adulthood, though as adults, this fear of the unfamiliar might manifest as social anxiety, generalized anxiety  or depression.

 They found that this was true to an extent among the boys in their group: When shown unfamiliar faces, the boys that had been high reactive infants tended to have stronger responses in the part of the brain that processes threat and novelty when compared to subjects who had been less fussy as infants.

 ”The idea is that when these kids walk into a room of strangers, their brains respond more,”  Dr. Carl Schwartz, the Massachusetts General Hospital psychiatrist who led the study, told ABCNews.com.  They are interpreting the situation as a threat, whereas an extrovert wouldn’t, he says. “These are the kids who are afraid to raise their hands in class, who don’t date in high school,” Schwartz says.

 ”I would never want to say that this is deterministic,” he adds.  It’s not that someone with a reactive temperament is doomed to be anxious or introverted, but these results suggest that it might be harder for a reactive infant to grow up into an  “extreme extrovert” because there is something happening early on that affects how their brains react, he says.

 This concept of inborn personality or temperament has been an issue of much debate among developmental psychologists over the past decades.  Post World War II , the going theory was that babies were like blank slates upon which the world would make its mark — it was nurture, not nature; there was no predicting who would be outgoing or who would be anxious by how they behaved as babies. 

In the past few decades, however, psychologists have started to pay  more  attention to the disposition babies express very early on in life, and how temperament may serve as a window  into a child’s future personality and mental health.

 But looking at only the temperament may take the “nature” side of the equation too far,  Jerry Aldridge, a professor of early childhood education at the University of Alabama at Birmingham, told ABCNews.com.

While the temperament may be inborn,  says Aldridge, it’s how that temperament is nurtured, or not nurtured, that determines whether a fearful infant might grow up into an anxious adult.

 ”The link between early temperament and propensity to psychological disorder later in life has a lot to do with the environment. Children whose environment supports their temperaments do better than those whose environment causes a ‘badness of fit’ between the child and the environment,” he says.

 For instance, children who are shy and fearful of new environments should be given extra time to adjust and have a caregiver around to make them feel safe, Aldridge says.  “We can’t just look at biology when working with young children.”