The study, published in the journal Sex Roles, showed that there were twice as many male as female main characters in the books examined and that male characters appeared in 53 percent more illustrations. Men, in general, were depicted in stereotypical jobs and frequently outdoors.
Not one image showed a father kissing, hugging or feeding a baby.
More women than men appeared to have no paid occupations in the books, and those occupations attributed to women were gender stereotyped, as were the occupations of men.
Studies since the early 1970s have documented such stereotypes in children's picture books. Little progress has been made in eradicating them in the last three decades, Anderson and Hamilton said.
Both of the authors are parents who read picture books to their children. Several years ago, it appeared to them that gender bias was "ingrained in those books," Anderson said. Many other parents may have noticed the bias, as well, he said.
"But I don't think they are aware of the extent of the bias," he said. "We were surprised when we actually quantified it. We would suspect some bias, but not to the extent that it exists."