AKA Social Selection Dynamic
Quick Definition: The changes in power and the way interactions occur between men and women when percentages of each sex change within the population of a geographic area.
I quote David Buss because there is no better explanation for this concept:
Men and women both are alert to deception at the hands of the opposite sex. Women holdout sexually, require the demonstration of honorable intentions and commitment, and penetrate possible deceptions to discover hidden commitments. Men conceal their emotions, disguise their external commitments, and remain uncommunicative and non-committal. They tried to abscond the sexual benefit without paying the cost of commitment.
The ratio of available women to men that affects the prevailing tactics used to attract a partner. The typical ratio at a singles bar, or example, favors women, because many more men than women are seeking short-term sex partners, and so those women who are looking for a brief encounter can exercise a great deal of choice. The sex ratio imbalance pressures men to best other men with better lines, better deceptions, and better simulations of the criteria that women impose. The losers typically outnumber the winners, and most men go home alone.
Where the sex ratio is reversed and there are many more available women than men, the balance of power shifts to men because they can more easily attract women for casual sex. This imbalance becomes especially pronounced in inner-city contexts, where crime convictions and homicides often take even more men out of the available dating pool. These conditions are extremely unfavorable to women who seek long-term mates, taxing their attraction tactics and rendering competition among women commensurately fiercer.
This trend is exacerbated by women’s high standards for mate: they’re choosiness dramatically shrinks the effective pool of eligible men. Many men are dismissed for failing to pass even the preliminary trials. This leaves just a few survivors — men of high status, high self-confidence, and high resource potential, over whom women then do battle. Those who succeed at attracting a lasting mate then face the next adaptive problem — staying together. [Title of the Next Chapter]
Hugh Hefner always throws parties that heavily favor the male selection dynamic at the Playboy Mansion:
Source: David Buss, in The Evolution Of Desire – Revised Edition 4