Stress impairs cognitive flexibility in infants.

In human adults, learning and memory under acute stress are characterized by an increased use of rigid habitual response strategies at the cost of flexible cognitive strategies. The immediate effects of stress on cognitive functioning early in life are not well understood. Here we show experimentally that acute stress leads human infants to perform habitual behavior rigidly. We found that 15-mo-old infants exposed to stress thereafter kept performing a previously effective action, even after the action suddenly became ineffective. Infants in a no-stress control group flexibly adjusted their behavior by disengaging from the newly ineffective action in favor of exploring an alternative action. This finding demonstrates that stress impairs infants' ability to adjust their behavior to changing circumstances.