“…Discovery that another has foregone short-term rewards in the pursuit of a long-term exchange relationship, for example by providing resource or assistance, presents a fortunate relationship building opportunity for the recipient. Appreciation or gratitude can signal one's favorable valuation of the other and pre-commitment or propensity to cooperate with them (Hirshleifer, 1987;Tooby & Cosmides, 2008), encouraging future trust much in the way that "promises" do (Schniter, Sheremeta, & Sznycer, 2013). Experimental evidence supports this functional account of appreciative and grateful feelings (Tesser, Gatewood, & Driver, 1968;McCullough et al, 2001;Dunn & Schweitzer, 2005;Algoe, Haidt, & Gable, 2008).…”