Boxplots of the average rate of yawning per minute during Baseline (white) and Stimulus (grey) for lipsmacking Non-imitators (top) and Imitators (bottom).

<p>LPS = lipsmacking condition, TP = tongue protrusion condition, CTRL = control condition. Boxplots depict 1^st and 3^rd quartile, line depicts median, whiskers depict 1.5 times inter-quartile range (IQR). Circles represent mild outliers larger than Q3 + 1.5 * IQR, asterisks represent extreme outliers larger than Q3 + 3 * IQR.</p

Boxplots of the average rate of yawning per minute during Baseline (white) and Stimulus (grey) for tongue protrusion Non-imitators (top) and Imitators (bottom).

<p>LPS = lipsmacking condition, TP = tongue protrusion condition, CTRL = control condition. Boxplots depict 1^st and 3^rd quartile, line depicts median, whiskers depict 1.5 times inter-quartile range (IQR). Circles represent mild outliers larger than Q3 + 1.5 * IQR, asterisks represent extreme outliers larger than Q3 + 3 * IQR.</p

Methodological details.

<p>Procedure (A), examples of lipsmacking, tongue protrusion, and control stimuli (B), and examples of infants' gestures (C). Human models portrayed here have given written informed consent, as outlined in the PLOS consent form, for publication of their photographs.</p

Scatterplots of the relationship between imitation strength and gesture rates during the Return period.

<p>Imitation strength is reported separately for lipsmacking (LPS; plots A and B) and for tongue protrusion (TP; plots C and D), and was calculated using the LPS and TP Imitation Index (see main text). Higher scores indicate stronger imitation. Only for LPS was there a stronger correlation when the return person was the same (A) compared to when the return person was different (B), <i>p</i><.001; for TP there was no difference in the correlation coefficients when the return person was the same (C) or different (D), <i>p</i> = .17.</p