A heartbreaking social statistic is that children on welfare have only about half as many words per day directed at them as the children of working-class families-- and less than one-third as many words as children whose parents are professionals. This is especially painful in view of the fact that scientists have found that the actual physical development of the brain is affected by how much interaction young children receive.
Even if every child entered the world with equal innate ability, by the time they were grown they would nevertheless have very different mental capabilities. Innate ability is the ability that exists at the moment of conception, but nobody applies for a job or for college admission at the moment of conception. Even between conception and birth, other influences affect the development of the brain, as well as the rest of the body.
The mother's diet and her intake of alcohol or drugs affects the unborn child. Differences in the amount of nutrition received in the womb create differences even between identical twins. Where one of these identical twins is born significantly heavier than the other, and the lighter one falls below some critical weight, the heavier one tends to have a higher IQ in later years. They may be the same weight when they become adults, but they didn't get the same nutrition back when their brains were first developing.
Inequalities have so many sources that this fact undermines the simple dichotomy between believing that some people are innately inferior and believing that discrimination or other social injustices account for economic and social differences. Yet people who are afraid of being considered racists, or believers that the lower classes are born inferior, often buy the notion that only the sins of "society" can explain why some people end up so much better off than others.
Decades ago, Edward Banfield pointed out how the different ways that children from different classes are raised helps or hinders them in their later life. Yet he was demonized by the intelligentsia for saying what most people would consider only common sense.