As a left of liberal gal, always wary of racism in the garb of Social Darwinism, this article on Social Darwinism from The American Conservative impressed me.
He considers the Darwinian implications of female infanticide and equal inheritance by male children. Two thousand years is long enough for strong cultural and biological selection pressures to impact a gene pool.
It's a long article. You might want to jump straight the last two paragraphs of "A People Shaped by Their Difficult Environment".
... the flip-side of possible peasant upward mobility was the far greater likelihood of downward mobility, which was enormous and probably represented the single most significant factor shaping the modern Chinese people. Each generation, a few who were lucky or able might rise, but a vast multitude always fell, and those families near the bottom simply disappeared from the world. Traditional rural China was a society faced with the reality of an enormous and inexorable downward mobility: for centuries, nearly all Chinese ended their lives much poorer than had their parents.
... only the wealthier families of a Chinese village could afford the costs associated with obtaining wives for their sons, with female infanticide and other factors regularly ensuring up to a 15 percent shortfall in the number of available women. Thus, the poorest village strata usually failed to reproduce at all, while poverty and malnourishment also tended to lower fertility and raise infant mortality as one moved downward along the economic gradient. At the same time, the wealthiest villagers sometimes could afford multiple wives or concubines and regularly produced much larger numbers of surviving offspring. Each generation, the poorest disappeared, the less affluent failed to replenish their numbers, and all those lower rungs on the economic ladder were filled by the downwardly mobile children of the fecund wealthy.
Furthermore, the forces of downward mobility in rural Chinese society were greatly accentuated by fenjia, the traditional system of inheritance, which required equal division of property among all sons, in sharp contrast to the practice of primogeniture commonly found in European countries.
... in China, cultural pressures forced a wealthy man to do his best to maximize the number of his surviving sons, and within the richer strata of a village it was not uncommon for a man to leave two, three, or even more male heirs, compelling each to begin his economic independence with merely a fraction of his father’s wealth. Unless they succeeded in substantially augmenting their inheritance, the sons of a particularly fecund rich landlord might be middle peasants—and his grandchildren, starving poor peasants.29 Families whose elevated status derived from a single fortuitous circumstance or a transient trait not deeply rooted in their behavioral characteristics therefore enjoyed only fleeting economic success, and poverty eventually culled their descendents from the village. The members of a successful family could maintain their economic position over time only if in each generation large amounts of additional wealth were extracted from their land and their neighbors through high intelligence, sharp business sense, hard work, and great diligence. The penalty for major business miscalculations or lack of sufficient effort was either personal or reproductive extinction.
... in the world of rural Chinese villages, even the wealthier elements usually spent the majority of the lives in backbreaking labor, working alongside their families and their hired men in the fields and rice paddies. Successful peasants might benefit from a good intellect, but they also required the propensity for hard manual toil, determination, diligence, and even such purely physical traits as resistance to injury and efficiency in food digestion. [emphasis mine]