ALMOST no one thinks it’s a good idea for unwed teenagers to become parents. It would be the odd parent, indeed, who counseled their own teenage son or daughter to start a family. Most parents hope that their children will finish school, find a job, and marry before they take on the burdens of parenthood. But what the majority of parents, almost regardless of race or social class, want for their own children is not what we have. Instead, 40 percent of all girls in the United States become pregnant before their twentieth birthday, and one out of every five goes on to become a teen mother. The overwhelming majority of these young mothers are unmarried and end up poor and on welfare.