The research supports other work that has shown perceptions of physical attractiveness alter with levels of economic and physiological stress linked to lifestyle.

BBC: Stressed men drawn to heavy women

"We saw a steeper decline in heart rate and a faster physiological stress recovery when they were smiling, " even though the participants weren't aware they were making facial expressions, says Sarah Pressman, co-author of the study and an assistant psychology professor at University of California, Irvine.

WSJ: Stress-Busting Smiles

Cortisol is produced in order to help the body recover from stress and regain its physiological stability.

BBC: Stress hormone heart death link

The fact that chronic stress, whether psychological or physiological, can lead to serious health problems has long been appreciated.

FORBES: How Major Psychological Stressors Can Lead To Serious Health Problems

There are generally physiological changes when people are in a stress situation.

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Others who experience early adversity seem to stop responding to stress at all, posting little or no physiological reaction.

WSJ: How to Turn Bad Stress Into Good

So while the infants' internal physiological distress levels had not changed, their outward displays of that stress were extinguished by sleep training.

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