We know that women are socialized to be more emotionally expressive than men, but it turns out mothers play a particular role in enforcing this gender norm, according to new British research.

Researchers at the University of Surrey found that when mothers speak to their daughters, particularly in early years, their conversations contain more emotional words and content than when they are speaking to their sons. Mothers also used more emotional language than fathers, unconsciously providing a model for their children that reinforces gender stereotypes.

In the experiment, the researchers asked 65 Spanish mothers and fathers to participate in a storytelling task and a conversation about past experiences with their four or six-year-old children. The parents' use of language was recorded and analyzed, and the researchers tallied the number of words associated with emotion used in each conversation.

The researchers found that mothers used more emotional language in general than fathers, and that mothers used the most emotion-associated language when speaking to their four-year-old daughters. There were no gender differences found with the parents of the six-year-old children.

Then, the researchers tested the children's use of emotional language. Both the four-year-old and the six-year-old girls were exposed more frequently to words like "happy," "sad" and "worried," and the six-year-old girls were found to use more "emotional talk" than boys of both ages when conversing with their parents.

"Girls may be socialized early into a socioemotional orientation emphasizing emotional expressivity, making them more socially mature than boys," the researchers conclude.

The study's lead author, Dr. Harriet Tenenbaum, agreed that cultural stereotypes about emotion were behind this difference.

"Think about the most emotional person you know," Tenenbaum said in an email to the Huffington Post. "More than likely, you will name a woman rather than a man because it conforms to our stereotypes."

These stereotypes may not be good for young men. Socializing boys without expressive emotional language could have negative implications for their development and future academic success. "Given that emotion is related to peer popularity, mental health, and school success, it would be better for both boys and girls if parents used emotion words with sons and daughters," Tenenbaum said.

There is a downside to this cultural norm of female emotional expressivity. Women who express emotional vulnerability at work tend to be perceived as weak and unprofessional, and female leaders are often harshly criticized for displaying emotion.

Although the research was conducted on Spanish parents, Tenenbaum said the results likely apply to parents in other countries as well.

"In the U.S. ... parents are three times more likely to explain science to boys than to girls," Tenenbaum said. "This study shows that parents socialise children differently based on gender. Although parents say that they do not treat girls and boys differently and instead, argue that children are innately different, combining these studies demonstrates that parents socialise children into traditional gender roles (emotion for girls and science for boys) at a young age."