The evolution of stepfathers; what they invest in stepchildren

EDITORS: Father's Day is Sunday, June 18.

ANN ARBOR---Stepfathers invest significant resources in their stepchildren, according to a University of Michigan researcher presenting a study on the life histories of American stepfathers at the annual meeting of the Human Behavior and Evolution Society in Amherst, Mass., on June 9.

"Many studies portray stepfathers in a negative light," says Kermyt Anderson, an anthropologist at the U-M Institute for Social Research, the world's largest academic survey and research organization. "But my work documents that stepfathers invest significant amounts of both money and time in their stepchildren."

From an evolutionary perspective, the question is why. The assumption has been that parenting another man's child is maladaptive. But Anderson's research suggests just the opposite---helping to raise other men's children gives stepfathers mating benefits and opportunities for lifetime reproductive success that they would otherwise be unlikely to obtain.

For the study, Anderson used data from the U-M Panel Study of Income Dynamics, a longitudinal study of a nationally representative sample of U.S. men, women, and children that started in 1968. Of 5,409 marriages he analyzed, he found that in 686, or nearly 13 percent, the men became stepfathers, marrying women who already had children by other men.

In general, Anderson found that men who became stepfathers had lower levels of education and income, and were more likely to have been divorced and to have children of their own---all characteristics that lowered their chances of marrying again. He also found that stepfathers marry later than men who are never stepfathers, and over the course of their lives are married for about seven or eight years less than other married men.

Raising one or two stepchildren, he found, has no effect on a man's total lifetime fertility. But raising three or more stepchildren is associated with a significant decrease in fertility. Still, while men who are stepfathers have fewer genetic children than ever-married men who are not stepfathers, they have significantly more children than men who never marry, Anderson points out.

"Men who are ranked lower in the mating market---because they have less education, lower incomes, were previously married, or already have children---are less likely to marry," Anderson explains. "By marrying women who have had children by other men, they're able to get mates that they might not otherwise be able to obtain. If they help raise one or two stepchildren, they're just as likely as men with no stepchildren to have children of their own within the marriage. Over their lifetimes, however, they're likely to have fewer children because they marry later and have been married for fewer years."

Being a stepfather may have benefits for men, but what about their stepchildren? In a recent study of 1,300 Albuquerque, N.M., men, published earlier this year in Evolution and Human Behavior, Anderson divided the men's children into four groups: genetic children of current mates, genetic children of previous mates, stepchildren of current mates, and stepchildren of previous mates.

For this study, conducted at the University of New Mexico, the men were asked how much money they had spent in the last year on each child age 17 and under in a number of specific categories, including education, clothing, hobbies, allowance, medical expenses, and gifts. As expected, he found that men invested the most money in genetic children of current mates ($2,570 in 1990 dollars), and the least in stepchildren of previous mates ($156). But the difference between what they invested in genetic children of former mates ($1,888) and stepchildren of current mates ($1,861) was insignificant when he controlled for the effects of co-residence.

He also found that men spent about 20 hours a week with genetic children ages 5 to 12 of current mates, compared with about 16 hours a week with stepchildren and about 10 hours a week with genetic children of previous mates.

"The time and money men spend on young offspring may be affected by legal requirements," notes Anderson, "as well as by the kind of relationship they have with the child's mother. So the amount they spend on older offspring may actually be more reflective of parental preferences. Also, since many older children aren't living at home, what fathers spend on them doesn't include the effects of co-residence."

For adult children between the ages of 18 and 24, he found, the men actually reported spending more on stepchildren of current mates ($1,828) than on genetic children of former mates ($1,535).

"Maybe stepfathers aren't as involved in their stepchildren's lives as biological dads are," says Anderson. "But this work shows that they are contributing a lot, and that they are helping to raise a large number of children."

Funding for the Albuquerque study was provided by the National Science Foundation and by the W.T. Grant Foundation. Funding for the Panel Study of Income Dynamics is provided by the National Science Foundation.