Birth timing and behavioral responsiveness predict individual differences in the mother-infant relationship and infant behavior during weaning and maternal breeding.

There is a great deal of variability in mother-infant interactions and infant behavior across the first year of life in rhesus monkeys. The current article has two specific aims: (1) to determine if birth timing predicts variability in the mother-infant relationship and infant behavior during weaning and maternal breeding, and (2) to identify predictors of infant behavior during a period of acute challenge, maternal breeding. Forty-one mother-infant pairs were observed during weaning when infants were 4.5 months old, and 33 were followed through maternal breeding. Subjective ratings of 16 adjectives reflecting qualities of maternal attitude, mother-infant interactions, and infant attitude were factor analyzed to construct factors relating to the mother-infant relationship (Relaxed and Aggressive) and infant behavior (Positive Engagement and Distress). During weaning, late born infants were more Positively Engaged than peak born infants (ANOVA, P < 0.05); however, birth timing did not affect the mother-infant relationship factors Relaxed and Aggressive or the infant attitude factor Distress. During maternal breeding, early born infants had less Relaxed relationships with their mothers than peak or late born infants, higher Positive Engagement scores than peak or late born infants, and tended to have higher Distress scores than peak born infants (repeated-measures ANOVA, P < 0.05). In addition, Distress scores were higher during maternal breeding than during the pre- and postbreeding phases. Finally, multiple regression (P < 0.05) indicated that while infant behavioral responsiveness predicted infant Positive Engagement during the acute challenge of maternal breeding, qualities of the mother-infant relationship predicted infant Distress. These data suggest that birth timing influences the patterns of mother-infant interactions during weaning and maternal breeding. Additionally, infant behavioral responsiveness and mother-infant relationship quality impact infant social engagement and affect expression, respectively.