A former Wall Street quant sounds an alarm on mathematical modeling´a pervasive new force in society that threatens to undermine democracy and widen inequality. We live in the age of the algorithm. Increasingly, the decisions that affect our lives´where we go to school, whether we get a car loan, how much we pay for health insurance´are being made not by humans, but by mathematical models. In theory, this should lead to greater fairness: everyone is judged according to the same rules, and bias is eliminated. But as Cathy ONeil reveals in this shocking book, the opposite is true. The models being used today are opaque, unregulated, and uncontestable, even when theyre wrong. Most troubling, they reinforce discrimination: If a poor student cant get a loan because a lending model deems him too risky (by virtue of his race or neighborhood), hes then cut off from the kind of education that could pull him out of poverty, and a vicious spiral ensues. Models are propping up the lucky and punishing the downtrodden, creating a ´toxic cocktail for democracy.´ Welcome to the dark side of Big Data.