Getting fussy infants and children to consume a wider variety of foods -- including the often refused green vegetables, the spurned tomato and even meat -- probably has little to do with taste or smell and a lot to do with learning, says psychologist Linda Bartoshok.

The ability to taste is inborn and may be completely developed a few weeks before birth. It can also be shaped by what is known in psychology as taste aversion -- associating a coincidental experience with an unpleasant reaction to a certain food. Development of taste buds suggests that unborn babies may be able to taste substances floating in the amniotic fluid, Bartoshok suggests. By birth, a child may already have had "a considerable amount of taste experience," she says.

Because humans are omnivorous, they have a built-in "neophobia" -- or fear of new foods -- as a natural, protective device.

"We're very cagey about what we choose to eat," says Bartoshok.

Even one "conditioned taste aversion" is very powerful, she says. "It can happen with one trial [taste of food] and last for a lifetime."

Parents are unlikely to realize what is happening, particularly with infants who naturally throw up a lot.

"You could be trying a new food on a baby," she says, "the baby throws up and associates the nausea with the food," even though the illness had nothing to do with what the baby ate.

Children can also be put off by the texture or appearance of a new food. Tomatoes probably fall into this category for many children, says Richard Doty, director of the University of Pennsylvania's Taste and Smell Center. The consistency of meat also seems unpleasant to many very young infants.

Some picky children "may have a few genetic differences in taste" that affects what they will eat, Bartoshok says. For instance, green vegetables often contain compounds related to a chemical called PTC -- a bitter substance known by the chemical name phenylthiocarbamide. Two of every three Americans can detect PTC, and dislike the bitter taste.

"If you put dark green vegetables on the table, the child may be perceiving the bitter smell," Bartoshok says, which is why he or she refuses to eat the food.

Children's food preferences are "enormously dependent on the family environment," she says. "I've trained my children to eat all the foods I like, including spicy Szechuan Chinese food with hot peppers."