From kindergarten to college, from generation to generation, the annual yearbook photo session has become an icon of the American school experience, an awkward, stomach-churning rite of passage captured in a flash and preserved for the ages in wallet-sizes.

Now, a researcher at Washington University in St. Louis has used thousands of yearbook photos to pinpoint a milestone in adolescent development -- the age when girls begin smiling more often than boys.

"The greater tendency for girls and women to smile more than boys and men, at least in school yearbooks, begins between the ages of 9 and 12, is firmly rooted by age 14 and persists into adulthood," asserts David K. Dodd, Ph.D., the study1s lead author and a senior lecturer in psychology in Arts & Sciences.

To smile or not to smile -- a dilemma as old as the Mona Lisa -- is an age-old yearbook question. Visit one of the popular Internet sites offering yearbook photos of famous people and you1ll find the smiling faces of Dick Clark and Bette Midler along with much more serious, close-lipped images of Tina Turner and James Dean.

Famous now and/or nerd then, most of us might prefer that our yearbook photos remain hidden away in some dusty archives. But from Dodd1s perspective, the nation1s easily accessible trove of vintage yearbooks represents a unique and valuable data set, a real-life snapshot record of how several generations have faced one of life's universal and enduring rituals.

For his study of adolescent smiling behaviors, which appears in the current quarterly issue of The Psychological Record, Dodd and colleagues examined 15,414 student photos in the yearbooks of nine elementary schools, three high schools and two colleges spanning the period 1968 to 1997. Adult smiling habits were benchmarked by analyzing yearbook photos of nearly 1,000 school faculty and staff. In addition, the study checked for historical shifts in smiling habits by comparing a sampling of yearbook photos from one high school over a 25-year period.

Although no one knows exactly why women smile more than men, their tendency to do so has been well documented in a flurry of research conducted over the last two decades. No matter the social setting, adult women have been shown to smile more often and more enthusiastically than male counterparts -- a gender difference apparently non-existent among newborns, toddlers and pre-schoolers. Dodd's study is the first to pinpoint the transition period, the point in adolescent development when young girls begin to diverge sharply from boys in their propensity to flash a broad and unfettered smile.

"Our research suggests that girls begin smiling significantly more than boys as early as the fourth grade and that this gender difference widens considerably by the time students reach high school," Dodd said. In kindergarten, when gender roles are still emerging, smiling for the yearbook appears to be a roughly 50-50 decision, although smiling girls (59 percent) slightly outnumber smiling boys (54 percent).

Willingness to smile increases steadily for both boys and girls during grades one through four, although girls always maintain a small lead in the smiling derby. By fourth grade, 77 percent of boys sport yearbook smiles, compared with 89 percent of the girls, the first point at which the gender difference is statistically significant.

"Grades four through six appear to represent a significant turning point for smiling and other behaviors related to gender development," Dodd said. "It is surely not coincidental that this period corresponds to puberty and heightened interest in the opposite sex. By freshman year, the difference in smiling reaches its peak, with 70 percent of girls smiling, compared to only 43 percent of boys."

Interestingly, both boys and girls experience a small jump in their willingness to smile for senior class high school photos -- smiling climbs to 84 percent among girls and to 65 percent among boys. Similar percentages persist in college yearbooks where smiles are present on 87 percent of young women and 64 percent of young men. Among the mostly adult population of school faculty and staff, yearbook smiling rates average 56 percent for men to 80 percent for women.

By pinpointing the various ages at which gender differences in smiling emerge, researchers hope to gain a greater understanding of when and why various gender differences take hold among adolescents. Although a number of theories have been put forward to explain why women smile more than men, most focus on how preadolescents respond to cultural messages about sex and sexuality.

Dodd and others have shown, for instance, that women in newspaper and magazine ads are much more likely than men to be portrayed as weak, passive, frivolous and lighthearted. "If preadolescents turn to the media for their definitions of the �ideal' man and woman, they are likely to find stereotypical portrayals of serious, unsmiling men and lighthearted, smiling women," Dodd said.

An alternative explanation of the study's findings focuses on the social nature of the event of being photographed for a school yearbook. "Girls and young women may view 'picture day' as a unique social event, in which there is a social pressure to dress up and present their 'best face' for the camera," Dodd said. "They are socialized to go to great lengths to improve their physical appearance and they may view picture day as an important opportunity to look good not only for their school mates and families but even for future generations who will view the yearbook."

Boys, on the other hand, may see a smile as less then essential to putting their "best face" forward, prefering instead to project an image of seriousness, an important characteristic of masculinity, Dodd said. Regardless of motivation, the smile's place in yearbook photos appears to be secure. Despite a general loosening of societal standards regarding gender roles, Dodd's analysis found that smiling habits of the sexes have changed very little in nearly three decades of yearbook photo sessions.

"It's possible that there have been broad changes in smiling behaviors in less formal social settings over the years, but the habits of students sitting for yearbook photos seem particularly resistant to cultural change," Dodd said.

Note: For information, refer to Dodd DK, Russell BL, Jenkins C, "Smiling in School Yearbook Photos: Gender Differences From Kindergarten to Adulthood," The Psychological Record, 1999, 49, 543-554.