Despite a breast-feeding brouhaha kicked off last week by a Time magazine cover photo of a mom nursing her 3-year-old son, that's actually the norm worldwide, experts say. But in the United States, breast-feeding children that old is practiced among a tiny sliver of mothers.

Some online are calling it "perverted" and "dangerous" to nurse a 3-year-old, but "it's normal for our species," says Katherine Dettwyler, a professor of anthropology at the University of Delaware in Newark.

"It's not perverted, it's not sex, it's not women doing it for some perverse need," she says. "It's normal like a nine-month pregnancy is normal."

Dettwyler, who has published studies on breast-feeding, found that most children around the world are breast-fed for three to five years or longer.

That's a sharp contrast with babies in the United States. Numbers for 2011 show that about three-quarters of American babies are breast-fed at birth. By 6 months old, 44% are still being breast-fed, and by 12 months just 24% are, says Laurence Grummer-Strawn, chief of the Centers for Disease Control and Prevention's nutrition branch.

The number of moms who breast-feed two years and beyond in the United States isn't known because the data come from a survey done of 18-month-old babies. But Ruby Roy, a pediatrician at La Rabida Children's Hospital in Chicago, says that it's more common than might be believed, and that moms are just hiding it.

"There's so much negative social attitude that we just can't know," Roy says. "But I have had many women in my practice tell me that they are breast-feeding to two or three years. They're doing a night nursing before the baby goes to bed, or in the morning — but they're not going to tell anyone."

The World Health Organization recommends breast-feeding "up to two years of age or beyond." The American Academy of Pediatrics recommends that "babies should continue to breast-feed for a year and for as long as is mutually desired by the mother and baby."

When Dettwyler studied 1,280 U.S. children whose mothers nursed them for more than three years, she found they were "perfectly fine and they didn't need therapy and they didn't think they were having sex with their mothers."

The children were nursed between three and nine years, with half being weaned between ages 3 and 4. The mothers tended to be middle- and upper-class women, the majority of whom were highly educated and worked outside of the home.

"This is not the stereotype of the Earth Mother nursing the child until he's 5, and she also grows her own cotton and weaves her own diapers," Dettwyler says.

Multiple studies show that breast-feeding is beneficial for both mother and infant. Breast milk contains immune factors that protect children against infection while their own immune system is still developing.

There also appears to be a programming effect on the body such that babies who nursed have lower rates of disease long after they are weaned.

Overall, studies have shown that breast-fed babies have lower rates of ear infections, eczema, diarrhea, lower respiratory tract infections, sudden infant death syndrome, obesity, leukemia and childhood diabetes.

Mothers who breast-feed have lower rates of breast cancer and ovarian cancer, says Grummer-Strawn. The longer they breast-feed, the lower their rates, he says.

It's also possible that we evolved to nurse children until they're around 5 or 6, says Dettwyler. Breast milk is one of the only sources of long chain polyunsaturated fatty acids that build brain tissue, she says.

It isn't until age 5 or 6 that "95% of brain growth has been reached, and that's also about the time that the child's immune system is ramped up to full production," she says.