More than half of all American children will likely live with an unmarried mother at some point before they reach age 18, according to a report issued by Princeton University and Harvard University. The absence of a biological father increases the likelihood that a child will exhibit antisocial behaviors like aggression, rule-breaking and delinquency, the researchers report in the journal EducationNext. This finding -- which holds true regardless of a child's race -- is especially prevalent among young boys. As a result, these children are 40 percent less likely to finish high school or attend college.

Researchers Sara McLanahan of Princeton's Woodrow Wilson School of Public and International Affairs and Christopher Jencks of Harvard wrote their report to coincide with the 50th anniversary of the controversial "Moynihan Report," a 1965 study by sociologist Daniel Patrick Moynihan (who later served as a New York senator) that argued that growing up in homes without a male breadwinner led to a life of poverty, unemployment and crime, especially for African Americans. McLanahan and Jencks are among some of the first researchers to examine the trends Moynihan projected since his report was furiously denounced in the '60s.

The researchers found that since 1965, the percentage of children raised by unmarried mothers has risen from 25 to 50 percent among blacks, and 7 to 19 percent among whites. ("Unmarried" mothers are defined only by marital status, not whether the mother lives with a partner.)

However, the racial makeup of single-mother families has not changed much over time. In 1970, 31 percent of single-mother families were black, 68 percent were white and 1 percent were "other race." In 2013, the figures were 30 percent black, 62 percent white and 8 percent "other." Evidence on the impact of these trends comes from the Fragile Families and Child Wellbeing study, pioneered by McLanahan, which is following a cohort of nearly 5,000 children born in large American cities between 1998 and 2000.

In the past five decades, the meaning of single motherhood has changed dramatically, McLanahan and Jencks write. Single mothers today are far less likely than their predecessors to have ever been married. Now, single motherhood usually occurs earlier in a child's life, or even at the very beginning. It is not uncommon for women to be single when their first child is born. Also, the high rate of partner turnover during a mother's peak fertility years means that children now experience multiple men entering and exiting their lives.

"Both the departure of a father and the arrival of a mother's new partner disrupt family routines and are stressful for most children, regardless of whether the father was married to the mother or just living with her," said McLanahan, director of the Bendheim-Thoman Center for Research on Child Wellbeing at Princeton's Woodrow Wilson School of Public and International Affairs. "Likewise, this shift to never-married motherhood has probably weakened the economic and emotional ties between children and their absent fathers."