Sex differences in motor activity during early infancy

Maturational and situational determinants of reaching at its onset

At 3 months of age, reaching behavior was measured in a group of 10 girls and 10 boys born at term. The assessments were carried out on the average 2 days after reaching onset. Reaching kinematics was measured in both supine and reclined positions. Girls reached more than boys, had straighter reaching trajectories and movements of shorter durations as well as fewer movement units. The reclined position gave rise to straighter trajectories in both girls and boys. Several anthropometric parameters were measured. Girls had less length and volume of the forearm than boys but similar upper arm volumes. There was a weak relation between kinematic and anthropometric variables.

Five-month-old girls' and boys' exploratory behaviors in the presence of familiar and unfamiliar toys

The exploratory behaviors of 60 5-month-old girls and boys were observed as they played with three unfamiliar toys and one familiar toy of their own during a laboratory session. Infants were seated at a table on the lap of an assistant. Each toy was placed on the table and remained there for 2 min after the infant first touched it. Frequency and duration of visual fixation on the toy, hand contact with the toy, five categories of lateralization, and eight categories of handling behaviors were measured. On the whole, the exploratory behaviors of girls and boys were not markedly different. However, girls spent more time than boys did in simultaneous hand contact with and visual fixation on their own familiar toy, whereas boys made more frequent contact with the familiar toy without visual fixation. In addition, boys displayed, overall, more tapping behaviors, and girls, more pinching and object-displacement behaviors. Girls and boys did not differ in manual lateralization. Results are discussed in reference to infants' rate of development, motor activity level, and girls' and boys' reactions to novelty.