Three bio-cultural features of existing humans must have had an evolutionary history. These features are (1) a reduced sexual dimorphism, (2) the systematic non-random paternal provisioning of particular women and children, and (3) human vulnerability to a wide array of sexually transmitted diseases (STDs). It is argued that these three features may have been interrelated for a long time. Fisher's model of the sex contract - an exchange of sexual exclusivity for preferential provisioning & protection - is reviewed within the dual contexts of (i) the transition from a multiple partner reproductive strategy to a pair-bonding reproductive strategy and (ii) the negative impact of sexually transmitted diseases upon human fertility. Key Words: Sex contract, reproductive strategies, pair-bonding, human evolution, sexually transmitted diseases Homo sapiens has three unique features which would be unexpected from a large, terrestrial primate: (i) its sexual dimorphism is minimal, (ii) the men systematically and actively provision particular women and the men's own children, and (iii) the species harbors a wide array of sexually transmitted diseases (STDs) which can severely and adversely affect reproductive health of individuals and, by extension, of the group. This article attempts to lend insight into how these three features may have interacted with each other in our evolutionary history. (i) Reduced sexual dimorphism Although there are exceptions, sexual dimorphism (e.g. by weight [from Hall, 1985]) tends to be greater in (semi)terrestrial primates than in arboreal primates (baboon [Papio 185] vs. spider monkey [Ateles 94], gorilla [Pan gorilla 219] vs. langurs [Presbytis 107]). Again with exceptions, larger primates tend to have more sexual dimorphism than smaller primates (orangutan [Pongo pygmaeus 199] vs. night monkey [Aotus trivirgatus 102]). Sexual dimorphism also tends to be lesser or nonexistent in primates which tend to be monogamous (gibbon [Hylobates 104] vs. chimpanzee [Pan troglodytes 121]; marmoset [Callithrix jacchus 95] vs. macaque [Macaca 149]). Arguably, when the males exchanged their ecological niche from the harder to scan world of the trees to that of the ground, they were better able to have and to maintain multiple sexual partnerships. Indeed, although there is certainly over-lap (e.g. see Garber, 1997), terrestrial primates are more prone to be polygynous than are arboreal primates. Accordingly, after it was freed from problems of fissile tree limbs & incessant gravity, additional male size would be advantageous in creating dominance for the larger male and in creating submission in the smaller male (see Fleagle, 1988; Martin, Willner & Dettling, 1994; McHenry, 1991a; Richard, 1985 for examples and discussion). Hence, a more effective male-to-male dominance displays/aggression could then be translated to multiple partners. These multiple partnerships would lead to a greater number of descendants who, in turn, would pass on the genetic material underpinning the physical attributes of the successful display. The same argument would apply to canine size and piloerection or other items (e.g. manes) which could be used to gain dominance and, thereby, more sexual partners, and, hence, more descendants. There are four givens which are germane here: (a) Homo's predecessor, Australopithecus, may have exhibited a large amount of sexual dimorophism by size (see Hall, 1985; Plavcan & van Schaik, 1997 for discussion), and (b) Homo, compared to Australopithecus, gradually increased in size (Hall, 1985; Aiello, 1994), and (c) Homo became exclusively terrestrial and (d) terrestrial primates bias away from monogamy. These four givens would, not unreasonably, infer that Homo would follow the basic trend of maintaining or increasing its sexual dimorphism. However, sexual dimorphism decreased (Economist, 1994; Lewin, 1989; Lockwood et al., 1996; McHenry, 1991b). …