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The interplay of environmental and biological factors is also demonstrated in research comparing the frequency of smiling in different countries and ethnic groups. This research shows that European American males and females differ more in their smiling rates than African American males and females, consistent with the finding that African American parents treat their sons and daughters more alike than do European American parents (LaFrance et al., 2003). Infants express their joy in giggles and gales of laughter as well as by smiling, and these expressions of emotion also change with age. Researchers have studied what makes babies of different ages laugh by showing them various stimuli or engaging them in different activities. Sroufe and his colleagues exposed 4- to 12-month-old infants to auditory stimuli, such as lip-popping, whispering, or whinnying sounds; tactile stimuli including bouncing the baby; visual stimuli including a human mask; and social stimuli, such as playing peekaboo (Sroufe, 1996). Auditory stimuli elicited few laughs at any age. Tactile stimuli elicited a substantial amount of laughter, but only at 7 to 9 months. Visual and social stimuli elicited more laughter overall, and the likelihood of this laughter increased with age. Need Assignment Help?