Evolution May Explain Aggressive and Meek Toddlers

Credit: © Photographer: Showface | Agency: Dreamstime.com CREDIT: © Photographer: Showface | Agency: Dreamstime.com

How a child responds to stress is written in their hormones, with tots falling into one of two types — those who meet stressful situations head-on and those who are more cautious, a new study finds.

These two different personalities are driven by opposing hormone reactions and probably have an evolutionary basis, challenging the idea that there is only one way to be mentally healthy and normal, the researchers say.

"Divergent reactions — both behaviorally and chemically — may be an evolutionary response to stress," study researcher Patrick Davies, of the University of Rochester, said in a statement. "These biological reactions may have provided our human ancestors with adaptive survival advantages."

Hawkish hormones

Davies and colleagues studied 201 toddlers from low-income families. They interviewed the parents and determined the level of disagreement and aggression the child witnessed between the parents. They assessed each child's personality type, watching them approach a new, potentially stressful, situation. Those who chose a meeker approach, the researchers called "doves," while those who approached the situation more aggressively were deemed "hawks."

Each child was exposed to a simulated, mildly stressful telephone conversation between his or her parents while the researchers tested the child's hormone levels. In the children whose parents fought often, the researchers saw two distinct patterns of hormone responses in the different personality types. 

Doves with parents who fought violently produced elevated levels of cortisol, a hormone thought to increase a person's sensitivity to stress. Hawks from such stressful home environments put the breaks on cortisol production, which is regarded as a marker for diminishing experiences of danger and alarm.

Divergent evolution

Individuals with heightened cortisol levels, like those with the dove personality, tend to be less likely than others to have attention problems, but more likely to develop anxiety and depression over time. By contrast, those with lower cortisol levels, like those found in the hawk types, are less likely than others to have anxiety problems, but are also more prone to risky behavior linked with attention and hyperactivity problems.

A meeker approach may work better under some challenging family conditions, while a more aggressive hawkish personality could be an asset in others. The researchers point out that this is an important counterpoint to the prevailing idea in psychology that "there is one healthy way of being and that all behaviors are either adaptive or maladaptive."

"When it comes to healthy psychological behavior, one size does not fit all," study researcher Melissa Sturge-Apple, also at the University of Rochester, said in a statement. She added that the findings "give us insight into how basic behavioral patterns are also chemical patterns."

The study was published July 8 in the journal Development and Psychopathology.

You can follow LiveScience staff writer Jennifer Welsh on Twitter @microbelover. Follow LiveScience for the latest in science news and discoveries on Twitter @livescience and on Facebook.