During the holiday season, gifts, cards, carols and donations constantly urge us to give thanks. But gratitude really can have beneficial psychological effects.

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Gratitude is one of those tricky, hard-to-pin-down feelings that can be either inert or powerfully transformative. At this time of year, when we’re constantly importuned to give thanks with gifts, cards, carols and donations, it often becomes a reflex. So when does gratitude have psychological effects?

That question didn’t get much scientific scrutiny until recently. Only in the past decade has there been a push to determine if gratitude “decreases pain and depression, and boosts happiness,” as a recent study in Primary Health Care Research & Development put it. The researchers found that an act of explicitly expressing gratitude lifted people’s mood and sense of well-being.

Bolstering this finding, other targeted studies have shown that health-care workers who cataloged why they were grateful experienced a 28% reduction in stress, and that writing about gratitude halved the risk of depression in those with a history of the disease.

Some research results seem almost too good to be true. Simply asking suicidal patients to write a letter of gratitude reduced their hopelessness in 90% of the cases. Among fit teenage athletes, those with high levels of gratitude were more satisfied with life in general and with their teams in particular.

Counting one’s blessings, as opposed to life’s annoyances, seems to bring with it all kinds of benefits: resilience, better health, a rosier outlook—even a longer, more restful night’s sleep and a sense of connectedness to other people.

Changing how we feel is one thing, but changing behavior is another. “It’s not the hardest sell in the world that emotions could make you feel more optimistic about your life. But I grow skeptical about observable effects in the body,” Michael McCullough told me. A psychology professor at the University of Miami who investigates emotions like forgiveness, revenge and gratitude, Dr. McCullough wonders whether feeling grateful actually alters our health or whether it works by motivating us to change our behavior—to quit smoking or drinking, for example.

He found both mood and behavior changes in an experiment he did with Robert Emmons, a colleague at the University of California, Davis. In the study, reported in 2003 in the Journal of Personality and Social Psychology, prompting people to list five things they were grateful for several times a week not only brought an uptick in mood but also resulted in subjects devoting more time to exercise and to helping others.

“Gratitude motivates people into trying to give back,” Dr. McCullough said, “and the research is really good that volunteering is good for health. Emotional state to social contact to feeding back into health behavior—it all makes sense.”

Whether the feeling or the behavior comes first, we do know that gratitude is tied to conscientiousness. Grateful people eat 25% fewer fatty foods and have better blood pressure readings than ungrateful folks. And a new, still unpublished study shows that feeling thankful is linked to lower Hemoglobin A1C, a sign of good blood-glucose management and thus better diabetes control.

In fact, gratitude is such a powerful catalyst for feeling hale and hearty, it’s a wonder that no one (except greeting-card companies and religious leaders) has found a way to package it.

Which brings us back to the holidays. This time of year most of us steel ourselves for bland turkey, rich desserts and loud relatives and forget what is remarkable in our lives. We’d probably all be better off—and enjoy the holidays more—if we followed the lead of gratitude researchers and took a few minutes to list exactly what we’re grateful for. Better still, think it over, then say it out loud so that everyone can hear it.