Natural selection is driven by the competition among genes to be represented in the next generation. Reproduction leads to a geometric increase in descendents, and on a finite planet not every organism alive in one generation can have descendents several generations hence. Therefore organisms reproduce, to some extent, at one another's expense.... Everyone alive today is a descendent of millions of generations of ancestors who lived under these constraints but reproduced nonetheless. That means that all people today owe their existence to having winners as ancestors, and everyone today is designed, at least in some circumstances, to compete. -- Pinker, p. 427.

... design features that promote both direct reproduction and kin reproduction, and that make efficient trade-offs between the two, will replace those that do not. -- Barkow, p. 53

Now, if an organism is asexual, once the pathogens crack the safe of its body they also have cracked the safes of its children and siblings. Sexual reproduction is a way of changing the locks once a generation. By swapping half the genes out for different half, an organism gives its offspring a head start in the race against the local germs. Its molecular locks have a different combination of pins, so the germs have to start evolving new keys from scratch. A malevolent pathogen is the one thing in the world that rewards change for change's sake. -- Pinker, p. 462

Each of our romantic histories goes back only a few years, but the romantic history of our genes goes back millions. We are here only because our genes enjoyed an unbroken series of successful sexual relationships in every single generation since animals with eyes and brains first evolved more than one billion years ago. In each generation, our genes had to pass through the gateway called sexual choice. Human evolution is the story of how that gateway evolved new security systems, and how our minds evolved to charm our way past the ever more vigilant gatekeepers. -- Miller, p. 32

... we were neither created by an omniscient deity, nor did we evolve by blind, dumb natural selection. Rather, our evolution was shaped by beings intermediate in intelligence: our own ancestors, choosing their sexual partners as sensibly as they could. We have inherited both their sexual tastes for warm, witty, creative, intelligent, generous companions, and some of these traits they preferred. We are the outcome of their million-year-long genetic engineering experiment in which their sexual choices did the genetic screening. -- Miller, p.10

As we have seen across the animal Kingdom, one sex inevitably invests more in the production of offspring in the other sex competes for access to these choosy, committed parents. The greater the disparity of investment, the more dramatic the differences -- physical and behavioral -- between the sexes. -- Burnham and Phelan, p. 147

Mate choice is intrinsically discriminatory and judgmental, built to rank potential mates by reducing their rich subjectivity to a crass list of physical, mental, and social features.  It scrutinizes individuals for infinitesimally harmful mutations and trivial biological errors, anxiously anticipating any heritable weakness that natural selection would have spurned in the Pleistocene.  It discounts everything that humans have in common, focusing only on differences.  And it pays the most attention to the fitness indicators that amplify those differences to the greatest extent. -- Miller, p. 432

...refinement of species' characteristics continues beyond the basic design through sexiness, the ability to secure mating opportunities.  If you can produce more offspring or offspring of better quality, whatever features make this possible will be passed on the the next generation.  This leads to some strange results, features that would seem to have no survival value....If you are strong and healthy enough to carry a big handicap, it is likely you have good breeding potential and therefore will have "fit" offspring.  Your good genes will be passed on...It has been reasoned that this idea explains much human behavior.  We spend inordinate amounts of time and energy making ourselves look good in various ways.  Painstaking self-decoration, lives devoted to refining skills beyond our immediate survival requirements, ceaseless work, and competition to acquire ever greater wealth -- sometimes to the point of being self-defeating--are visible all around us. -- Nicholson, p. 106 (pocket ed.)

A fundamental principle of modern Darwinism is the idea that men and women have a common interest in passing on their genes, but quite different strategies for achieving this goal. -- 331.

Sex differences are a problem in organizations not because men and women are creatures from different planets--Mars and Venus--but because we overlap so much in almost all the ways we differ. -- Nicholson, p. 326.

... human culture and social behavior is richly variable because it is generated by an incredibly intricate, contingent set of functional programs that use and process information from the world, including information that is provided both intentionally and unintentionally by other human beings. -- Barkow, p. 24