There've been a lot of stories about girls losing their self-esteem when they become teenagers. Psychologist and Harvard University professor Carol Gilligan says the erosion of self-esteem often stems from girls' fear of expressing their own strengths and opinions.

Gilligan has been studying girls' behavior since the early '80s. She started a research group called the Harvard Project on Women's Psychology and Girls' Development, which she said was the first group to research and show the crisis in girls' lives at adolescence."What was surprising to us as adult women was the strength of young girls," Gilligan told Children's Express. "It was really fascinating because the girls' voices were familiar to us; we had been girls. But it was also surprising because it didn't fit in with the psychology we knew as psychologists."

Gilligan published the Harvard Project results in a number of research papers and books including: "In a Different Voice: Psychological Theory and Women's Development" and "Meeting at the Crossroads: Women's Psychology and Girls' Development."

Basically, the research group discovered that young girls do have high self-esteem, but as they grow older they get scared they might lose their friends if they express their opinions, so they end up trading their self-esteem to keep these relationships. Gil-ligan called this "going underground."

"By looking and listening and paying attention to the human world as girls were growing up and becoming teenagers, we saw that they were making a series of observations that were often very astute about the world around them, but a lot of what they were seeing and observing were things people didn't want to know they knew," she said.

"We did some sentence completion tests where one sentence starts, `What gets me in trouble is . . .' and a lot of girls say, `My big mouth.'

"If girls just speak about what they know, they reveal a human world that people generally don't want to engage with. So, actually, it is their strengths that get them into trouble and that is what we documented."

Gilligan said girls feel they'll be alone if they speak out because young girls are very cliquey. If the popular girls act a certain way, then other girls feel they should act that way, too.

"If girls feel they have to fit into a particular image or not be accepted, and if that image is very constraining that you can only say certain things or have certain feelings, then they may find themselves left out," Gilligan said. "It's very painful to be left all alone and to have no friends and no relationships."

One of the reasons this happens, said Gilligan, is that people expect different things from boys and girls. Parents treat boys and girls differently. Gilligan said girls were more protected and encouraged to have relationships instead of being strong and aggressive. Society confirms these messages, so stereotypes play a part in girls' psychological development.

Gilligan pointed out that when people do studies on children, they usually go to boys first or they never even look at girls. Also, she said that a lot of boys have a lack of understanding and a lack of interest in girls' lives. The fact that four of the five people who volunteered to work on this story are girls supports her theory.

To change this, Gilligan said that all of society has to change. "There used to be a time where in school almost no books that were about women's lives were read, and no history of women was taught," Gilligan said. "Until recently, people weren't aware of the kind of images of women they were taking in."

Gilligan said things like Take Our Daughters to Work Day, sponsored by the Ms. Foundation for Women, are helpful in showing people how important it is to talk about girls and to listen to them. She said things are changing, and someday we might see more female politicians or women in positions of power.

Before this can happen, Gil-ligan said girls have to learn that they can express their opinions, do anything they want and still have friends. "What I tell girls is to find people where you can speak your opinions and stay in the relationship," she said. "That gives you a place where you can explore relationships with other people, and also speak out in a way that's tied to your own experience."