This metaphor, from inequality writer Chuck Collins, captures something about the way that advantage and disadvantage get compounded:
Imagine a ten-mile race in which contestants have different starting lines based on parental education, income, and wealth. The economically privileged athletes start several hundred yards ahead of the disadvantaged runners. Each contestant begins with ten one-pound leg weights. The race begins, and the advantaged competitors pull ahead quickly. At each half-mile mark, according to the rules, the first twenty runners shed two pounds of weights while those in the last half of the field take on two additional pounds. After several miles, lead racers have no weights, while the slower runners carry twenty additional pounds. By midrace, an alarming gap has opened up in the field, and by the finish line, the last half of the field finishes more than two miles behind the winners.