People Change People is informed by research exploring how our relationships, words, and actions influence our health, well-being, and one another.

Harvard Study of Adult Development (1938–present)

https://news.harvard.edu/gazette/story/2023/02/work-out-daily-ok-but-how-socially-fit-are-you/
Kumar & Epley (2018), Psychological Science

Paper: https://doi.org/10.1177/0956797618772506

Readable summary: https://news.uchicago.edu/story/people-underestimate-value-sending-letters-appreciation

Participants underestimated how surprised and happy their gratitude would make others, and overestimated how awkward recipients would feel.
Holt-Lunstad, Smith & Layton (2010), PLoS Medicine

https://journals.plos.org/plosmedicine/article?id=10.1371/journal.pmed.1000316

Across 148 studies with 308,849 participants, people with stronger social relationships had a 50% increased likelihood of survival. The authors note that this effect is comparable to well-established risk factors like smoking and alcohol consumption.
Seligman et al. (2005), American Psychologist

https://pubmed.ncbi.nlm.nih.gov/16045394/

In a randomized, placebo-controlled study, the authors tested five happiness interventions and found that three lastingly increased happiness and decreased depressive symptoms. The gratitude visit was one of the five; its boost faded after about a month.