Our ultimate goal is reproduction, but evolution requires only that we are aware of our proximate goals, which, in order of priority, are: 1) sex, 2) intra-sexual competition, and 3) survival.

Men desire fertile, non-pregnant and faithful women. Women have a limited period of fertility, so men are attracted to youth, slenderness, beauty and chastity (Buss et al., 1990). Men want (in order of priority): 1) sex with many new, short-term partners, with the hope that other men will support their illegitimate children; and 2) exclusive sexual access to a long-term partner. The only way a man can achieve 1) is to attain status via his rank in the dominance hierarchy, whilst 2) allows a compromise between his status and reliability. A man's rank in the dominance hierarchy is correlated with height, muscularity, broad shoulders, confidence, symmetry, attractiveness, intelligence, wealth, income and willingness to take risks.

Women want (in order of priority): 1) a long-term provider/husband who will invest food and care in her children; and 2) a high-status lover who can give those children first-class genes. 1) and 2) can be the same man, but a woman is only likely to achieve this if she is highly desirable (Ridley (1993) p. 244). For long-term relationships, women seek high-status men as a primary consideration, and reliability as a secondary consideration. For short-term relationships, women seek only high-status men.

In summary, men and women both want short-term and long-term partners, but the priorities are the opposite way around. As the more discriminating sex it is women who choose, and therefore the men who must develop a strategy. The women’s priorities dictate the probability of the strategy working, whilst the man’s preferences determine the payoff. For men, 1 is a high risk/high return strategy, whilst 2 is a lower risk/lower return strategy.

For a monogamous species to maintain an evolutionary stable strategy, cheating is inevitable. Humans are almost, but not quite, monogamous.
GF = genetic fitness
cheating man gains: (his GF + cheating woman's GF)/2
cheating woman gains: (her GF + cheating man's GF)/2 - (her GF + husband's GF)/2
= (cheating man's GF - husband's GF)/2
The above implies that the majority of men will cheat, but a woman will only cheat if her lover has a higher genetic fitness than her husband.

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