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The sex ratio at birth is slanted towards males in humans by default, as there are more male foetuses conceived and born to full term than female offspring. Male immigranttion and male migration within a country may also directly increase the surplus of males.
This near-universal male preference for younger women would be expected to lead to greater competition among men for the relatively scarce young, fecund women in a particular population.
Research into the effects of OSR on mating behavior indicate that the effects of an imbalanced sex ratio are chiefly exerted via an imbalanced OSR serving to make the rarer sex in a particular environment either more choosy in terms of partner choice (with the more plentiful sex more prone to compromise in terms of standards) or more prone and capable of executing their favoured sexual strategies (argued to be uncommitted sex for males and relationships with men of high mate value for women, in line with parental investment theory).
Population aging and declining birth rates in many industrialised countries would be further expected to exacerbate this phenomenon, as this aging results in much fewer fertile age women as a portion of the population at large.
Sources
en.m.wikipedia.org