Can writing "thank you" notes to yourself make you happier? In one study, college students wrote once a week for ten weeks. One group wrote up to five things they were thankful for.

Another group wrote about things that annoyed them. A third group wrote about everyday events. The thankful group felt better about their lives. They even exercised more!

But the effect is usually small. It is not magic.

Keeping a gratitude journal sounds simple, but does it actually work? In a well-known 2003 study, Robert Emmons and Michael McCullough divided college students into three groups. For ten weeks, they wrote once a week about either things they were grateful for, daily hassles, or neutral events.

After ten weeks, the gratitude group felt more positive about their lives, had higher hopes for the coming week, reported fewer physical complaints, and even spent more time exercising than the hassles group. One explanation is that our brains naturally pay more attention to bad things, and writing about gratitude trains us to notice good things too. Another research team reported that writing once a week worked better than three times a week.

Gratitude journals are often promoted as a simple path to happiness. In a 2003 study, Robert Emmons and Michael McCullough randomly assigned college students to write weekly, for ten weeks, about up to five things they were grateful for, about daily hassles, or about neutral events. Compared with the other groups, the gratitude group rated their lives more favorably, felt more optimistic about the following week, reported fewer physical symptoms and spent more time exercising than the hassles group.

A plausible mechanism is that our attention is naturally biased toward negative events, and listing blessings redirects it toward positive ones and toward the people who support us. Frequency may also matter: Sonja Lyubomirsky and colleagues reported a benefit for writing once a week but not three times a week, perhaps because frequent writing becomes routine.

However, the evidence calls for modest expectations. The outcomes in the original study, including exercise time, were self-reported. Later meta-analyses found small to moderate effects compared with doing nothing, but much smaller effects when gratitude exercises were compared with other positive activities, and only small effects on depression and anxiety. Gratitude writing is therefore best seen as a cheap, helpful habit rather than a cure.