More than half of the babies and toddlers showed signs of previous abuse, according to a study of 72 deaths of children under age 2, published online this week in Pediatrics. The study included data from five states and seven cities.

Babies this age, who are vulnerable in so many ways, are also among the most likely of all children to be victims of homicide, the study says.

More than 2,400 children under 2 were murdered in the USA from 2001 to 2005, almost twice the number killed in car accidents, the study says. Children this age account for about half of all homicides of children under 14. The murder rate for babies this age — 6 per 100,000 children — is 10 times higher than the rate for children 7 to 8 years old, and even higher than the rate for 15- and 16-year-olds, the study says.

In the study, the largest number of babies died after being shaken to death, often because of crying.

Many of these deaths may be preventable, because evidence suggests that the perpetrators act out of frustration and ignorance rather than a desire to harm, says co-author Takeo Fujiwara the of Harvard School of Public Health.

The study also shows that child advocates need to do a better job of educating men about ways to cope with crying, says Carole Jenny, a spokeswoman for the American Academy of Pediatrics on child abuse.

More than 70% of perpetrators in the study were male. That's striking, given that women often spend more time with infants, says co-author Catherine Barber, also of Harvard.

There's no national system to educate parents about shaken baby syndrome before they leave the hospital, Jenny says. Although 14 states require that new parents receive information before leaving the hospital, states rarely provide money to fund these programs, says Philip Scribano, medical director of the Center for Child and Family Advocacy at the Ohio State University College of Medicine.

Although home visiting and hospital-based educational programs have had some success, many efforts fail to include fathers, he says.

"The message to fathers should be, 'Don't shake a baby,' " says Jenny, who notes that for every baby that dies from being shaken, seven or eight suffer brain damage, blindness or other injuries. "If you get angry, put the baby down and ask for help."