from a lateral perspective and showed two social agents, including only their hands and arms. The
stimulus presentation started with a still frame of a hand forming a fist that was positioned on the
right or left side of the screen and an object (white ball with colorful flowers) that was placed in
the middle of the scene. After approximately 100 ms, a second hand entered the scene from the side
(opposite to the first hand) and grasped and lifted the ball (average duration = 1875 ms). After squeezing
the ball accompanied by a sound (average duration = 1555 ms), the other hand subsequently
formed either a give-me gesture (average duration = 1700 ms) or an inverted hand shape (average
duration = 1600 ms) [Fig. 1A(I)]. After the ball was squeezed a second time (average duration
= 1625 ms) to ensure that infants attended to the object, the ball was passed to the receiving hand
on the other side of the screen (average duration in the Give-Me Gesture condition = 1680 ms; average
duration in the Inverted Hand Shape condition = 1720 ms) [Fig. 1A(II)]. The stimulus movie ended
with a still frame for 1 s showing the ball touching the receiving hand [Fig. 1A(III)]. All reaching actions
of the four different stimuli versions (alternating reaching from right or left side with either a give-me
gesture or an inverted hand shape as the goal) had approximately the same length (10.12, 10.24,
11.28, and 11.56 s, respectively).