In a recent study, researchers measured the level of the stress hormone cortisol in participants while they underwent interviews and did math problems in their heads. Ninety-five percent showed cortisol spikes. No surprise there. What was remarkable was that when people watched the participants through a one-way mirror, their cortisol rose, too, nearly a third of the time (even more when the person they were watching was their significant other). In fact, just seeing a video of the stressful situation made 24 percent of observers physiologically stressed.