In a 2005 paper, Seligman, along with Tracy Steen, Nansook Park, and Christopher Peterson, reported results from an extremely simple and cost-effective intervention to increase happiness. They conducted their study through an Internet Website. Visitors to the Website were invited to participate in exercises designed to increase their happiness, with a warning that some visitors would be assigned to a placebo condition. Placebo conditions are routinely used as a comparison against the active treatment condition. Study participants were assigned to perform one of the following exercises for one week only. In the placebo condition, subjects had to write about early memories each night.

There were five treatment conditions: the gratitude visit (within one week participants had to write and deliver a letter of gratitude to someone who had never been properly thanked for their kindness); three good things in life (each day participants had to record three good things that happened and consider what caused them); you at your best (subjects were asked to write about a time when they were at their best, identify their signature strengths at that time, and then reflect upon what they wrote every day for a week); using signature strengths in a new way (participants were asked to identify their top five character strengths and to put them to use in a novel way each day for a week); and identifying signature strengths (participants had to identify their top five signature strengths and to use them more often in the following week).

All of the conditions, including the placebo condition, improved happiness scores and reduced depression scores immediately after the exercises were completed. One week after that, scores for the placebo condition returned to baseline and stayed there for the next six months. The effect of “you at your best” and “identifying signature strengths” also disappeared within one week after the exercises ended. The effect of “the gratitude visit” lasted somewhat longer, persisting for a month. Remarkably, the effects of “the three good things” and “using signature strengths” interventions lasted for the entire six months of the study period. It appears that people who maintained improvement had continued performing their exercises throughout the six-month study period.