Spontaneous smile and spontaneous laugh: an intensive longitudinal case study.

One male infant was observed from the day of his birth to the end of the 6th month. Total observation days were 171 days, and total observation time was 329 h 25 min and 35s. Five hundred and sixty-five spontaneous smiles and 15 spontaneous laughs (smiles accompanied by vocal sounds) were observed. Developmental psychologists have thought that spontaneous smiles integrate at about 3 months, but spontaneous smiles were recorded even in the 6th month. The percentage of bilateral smiles increased from the 2nd month. This is the first intensive longitudinal study on spontaneous smiles and spontaneous laughs.