Two decades ago, a landmark study led by the psychologist Robert A. Emmons sought to understand how people benefit from gratitude, a question that scientists had rarely explored until then.

Dr. Emmons’s findings — which suggested that gratitude may improve psychological well-being — inspired a spate of additional research. To date, numerous studies have found that having a grateful outlook, “counting one’s blessings” and expressing gratitude to others can have positive effects on our emotional health as well as on interpersonal and romantic relationships.

“Gratitude heals, energizes and changes lives,” Dr. Emmons said. “It is the prism through which we view life in terms of gifts, givers, goodness and grace.”

Gratitude is a positive emotion that can arise when you acknowledge that you have goodness in your life and that other people — or higher powers, if you believe in them — have helped you achieve that goodness.

In other words, the sources of the good things “lie at least partially outside the self,” Dr. Emmons said.  You might feel gratitude when someone is kind to you, for example.

But “feeling it is only half the equation,” said Philip Watkins, a professor of psychology at Eastern Washington University.  Expressing gratitude is equally important to reap the benefits of this emotion, he said.

Many studies have asked participants to write letters of thanks, or to list the positive things in their lives, and then measured the effects of those acts.

The results suggest that performing these types of activities provides mental health benefits — reducing symptoms of depression and anxiety, increasing self-esteem and improving satisfaction with daily life.

And not only does gratitude improve the well-being of the giver and the recipient, but it may also be good for those who witness it: Watching an act of gratitude between two people can cause an observer to feel more warmth and affinity toward them both.

“What impresses me are the objective, biologically verifiable outcomes that go beyond self-report measures,” Dr. Emmons said. For example, gratitude has also been associated with lower blood pressure, and, in one pilot study, higher levels of heart rate variability, a marker of well-being.

“Gratitude seems to be the gift that keeps on giving,” Dr. Algoe said.

To develop an enduring gratitude habit, try linking your gratitude practice to an already ingrained routine, Dr. Wong said. He chooses to think about what he’s grateful for in the morning.

Gretchen Schmelzer, a psychologist in Philadelphia who regularly incorporates gratitude exercises into her work with clients, said it could be especially useful during difficult times. Earlier this year, she fell while hiking and broke both legs, leading her to use a wheelchair for six weeks.

To avoid spiraling into negative thoughts while she continues to heal, she tells herself each day to “be thankful for what you can do — and not let yourself focus on what you can’t do,” she said.  “Gratitude allows us to look at what we do have and to feel abundance,” she added.

Finally, although many studies have shown the value of writing a letter expressing appreciation, it doesn’t have to be lengthy or time-consuming. A quick email or text can do the trick.