It's that time of year when yearbooks are making the rounds. After two psychology researchers recently pored more than 18,000 student yearbook photographs, they made an interesting observation -- they found a gender difference in smiling among the classmates.

Between age 10 and 11, or by the time kids reached sixth grade, boys started smiling less than girls in their yearbook portraits, the new research found.

In the study, which was published online in the journal Sex Roles, researchers looked at snapshots of kids from kindergarten through twelfth grade taken between 1996 and 2008 at 17 schools in Michigan.

They focused on the faces in 34 different yearbooks -- zeroing in on the mouth -- to determine if there were any sex differences in smiling across the age groups. Each grin was rated on a 3-point scale with 0 given for no smile, 1 for a partial smile, and 2 for a full smile.

Boys and girls probably received the same "smile" or "say cheese" instructions from the photographer. But there was a noticeable drop off in grinning among the boys by sixth grade, while the girls kept on beaming for the camera.

Asked why boys may have slacked off on smiling, study author Dr. Mihaela Friedlmeier suggests that during their late tween and early teen years, boys and girls get plenty of information about gender-related expectations from parents, peers and the media.

They learn "how to assert themselves as distinctly female (emotionally expressive and friendly) or distinctly male (dominant, aggressive, and nonsmiling)" says Friedlmeier, an assistant professor of psychology at Grand Valley State University in Allendale, Mich.

Researchers also compared the smiling patterns in African American schoolchildren to European American kids. And they considered whether the ethnic diversity of the student body at each school mattered.

"We were surprised to find that African American boys displayed significantly less smile compared to European American boys," says Friedlmeier.

"It's particularly interesting that African American boys from schools with a predominantly African American student population showed less smile compared to those from ethnically mixed schools and those with a majority of European American students," she says. This was true even though there were no major socioeconomic differences in the students at the schools.

Friedlmeier thinks that African American teenage boys may have a different image of masculinity (as appearing "tough") compared to European American boys. And she says it's also possible that historical context might be influencing African American males. She suspects they may be avoiding the "Uncle Tom" image of the smiling Black man, popular in the 1940s and 1950s. Although the young boys clearly weren't alive then, this idea may be internalized and passed down from their parents or grandparents, as well as their peers and the media.

In this Michigan-based study, African American girls smiled a little less than their European American peers. But there may be fewer differences in female smiles because both groups are influenced by similar cultural standards of beauty, explains Friedlmeier.

Friedlmeier hesitates to extrapolate her findings in these Midwest schools to the rest of the country. But if you happen to thumb through your (or your kid's) yearbook, along with noticing the horrible hairtstyles and goofy quotes, see if the smiles vary not only by gender but also by ethnicity.

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