Selective investment theory proposes that close social bonds, and associated emotional, cognitive, and neurohormonal mechanisms, evolved to facilitate long-term, high-cost altruism between those closely depending on one another for survival and reproductive success.
sᵻlˈɛktɪv ɪnvˈɛstmənt θˈiəɹi pɹəpˈoʊzᵻz ðæt klˈoʊs sˈoʊʃəl bˈɑːndz, ænd ɐsˈoʊsɪˌeɪɾᵻd ɪmˈoʊʃənəl, kˈɑːɡnᵻtˌɪv, ænd nˈʊɹɹoʊhˌoːɹmɑːnəl mˈɛkənˌɪzəmz, ɪvˈɑːlvd tə fəsˈɪlᵻtˌeɪt lˈɔŋtˈɜːm, hˈaɪkˈɔst ˈɔːltɹuːˌɪzəm bᵻtwˌiːn ðoʊz klˈoʊsli dᵻpˈɛndɪŋ ˌɔn wˈʌn ɐnˈʌðɚ fɔːɹ sɚvˈaɪvəl ænd ɹᵻpɹədˈʌktɪv səksˈɛs.