Carly Moore sits at the grave site of her son, Jayden Cangro, on Oct. 30 in Salt Lake City. In July 2006, Carly's boyfriend, Phillip Guymon, threw the 2-year-old across a room because he didn't want to go to bed. Guymon is serving five years in prison for second-degree felony child abuse homicide.

New York ? Six-year-old Oscar Jimenez Jr. was beaten to death in California, then buried under fertilizer and cement. Two-year-old Devon Shackleford was drowned in an Arizona swimming pool. Jayden Cangro, also 2, died after being thrown across a room in Utah.

In each case, as in many others every year, the alleged or convicted perpetrator had been the boyfriend of the child’s mother – men thrust into father-like roles that they tragically failed to embrace.

Every family is different. Some single mothers bring men into their lives who lovingly help raise children when the biological father is gone for good.

Nonetheless, many scholars and social workers who monitor America’s families see the abusive-boyfriend syndrome as part of a broader, deeply worrisome trend. They note an ever-increasing share of America’s children grow up in homes without both biological parents, and say the risk of child abuse is markedly higher in the nontraditional family structures.

“This is the dark underbelly of cohabitation,” said Brad Wilcox, a University of Virginia sociologist. “Cohabitation has become quite common, and most people think, ‘What’s the harm?’ The harm is we’re increasing a pattern of relationships that’s not good for children.”

Statistics not clear

Existing U.S. data on child abuse is patchwork, making it hard to track national trends with precision. The latest federal survey on child maltreatment tallies nearly 900,000 abuse incidents reported to state agencies in 2005, but doesn’t delve into how abuse rates correlate with parents’ marital status or the makeup of a child’s household.

Similarly, data on the roughly 1,500 child abuse fatalities that occur annually in America leaves unanswered questions. Many of those deaths result from parental neglect, rather than overt physical abuse. Of the 500 or so deaths caused by physical abuse, the federal statistics don’t specify how many were caused by a stepparent or unmarried partner of the parent.

However, there are many other studies that reinforce the concerns. Among the findings:

¢ Children living in households with unrelated adults are nearly 50 times as likely to die of inflicted injuries as children living with two biological parents, according to a study of Missouri data published in the journal of the American Academy of Pediatrics in 2005.

¢ Children living in stepfamilies or with single parents are at higher risk of physical or sexual assault than children living with two biological or adoptive parents, according to several studies co-authored by David Finkelhor, director of the University of New Hampshire’s Crimes Against Children Research Center.

¢ Girls whose parents divorce face significantly higher risk of sexual assault, whether they live with their mother or father, according to research by Robin Wilson, a family law professor at Washington and Lee University.

Family dynamics changing

Census data makes clear that family patterns have changed dramatically in recent decades as cohabitation and single-parenthood became common. Thirty years ago, nearly 80 percent of America’s children lived with both parents. Now, only two-thirds of them do. Of all families with children, nearly 29 percent are now one-parent families, up from 17 percent in 1977.

The net result is a sharp increase in households with a statistically greater potential for instability, along with the likelihood that adults and children will reside in them who have no biological connection.

“I’ve seen many cases of physical and sexual abuse that come up with boyfriends, stepparents,” said Eliana Gil, clinical director for the national abuse prevention group Childhelp.

“It comes down to the fact they don’t have a relationship established with these kids,” she said. “Their primary interest is really the adult partner, and they may find themselves more irritated when there’s a problem with the children.”

That was the case with Jayden Cangro.

In July 2006, his mother’s boyfriend, Phillip Guymon, hurled the 2-year-old across a room in Murray, Utah, because he balked at going to bed. The child died as a result.

Jayden’s mother, Carly Moore, has undergone therapy since the killing. Yet she continues to second-guess herself about her two-year relationship with Guymon.

“There’s so much guilt,” she said in a telephone interview. “I never saw him hit my kids, ever. But he was gruff in his manner – there were signs that he wasn’t the most pleasant person for kids to be around.”

Guymon is serving five years in prison for second-degree felony child abuse homicide. Moore thinks the penalty is too light.

Boyfriend gets death

The slaying of toddler Devon Shackleford was premeditated. Derek Chappell, who was sentenced to death this month, considered Devon an obstacle to an on-again, off-again relationship with the boy’s mother, and drowned him in an apartment complex swimming pool in Mesa, Ariz.

There are, of course, initiatives aimed at reducing the percentage of children raised by single parents. That’s among the goals of the Bush administration’s Healthy Marriage Initiative.

“The risk (of abuse) to children outside a two-parent household is greater,” said Susan Orr, a child welfare specialist in the Department of Health and Human Services. “Does that mean all single parents abuse their children? Of course not. But the risk is certainly there, and it’s useful to know that.”