Left side cradling of an appetitive doll is associated with higher heart rate variability and attenuated startle in nulliparous females

Cradling represents a unique type of mother–infant interaction. A bias towards left side cradling of infants and baby-like dolls has been demonstrated in human females, irrespective of handedness. One explanation for this behavioural bias involves right hemispheric specialisation of decoding visual, acoustic, and tactile emotional signals of left perceptual field origin. This implies that emotional signals from the child could have a greater impact on the caregiver's affective state when originating in the left than right perceptual fields. This may represent a key reinforcing mechanism responsible for left side cradling, but this has never been tested. In the present study, sixty-two never-pregnant female volunteers held an appetitive baby-like doll on the left or the right arm while reflexive startle eye blinks to binaural acoustic noise probes, as well as heart rate variability (HRV), were assessed. During left side cradling startle eye blink magnitude was attenuated, and low frequency HRV was decreased while high-frequency HRV was significantly increased. Attenuation of startle occurs in positive affective contexts, and high-frequency HRV is a reliable marker of vagal activity known to increase in appetitive, non-stressful contexts. Thus, our results suggest that appetitive infant signals have more positive effects on affective processes of the female caregiver when presented in her left perceptual fields.