Familiarity to the mother and the novelty afforded by the postnatal environment are two contrasting sources of neonatal influence. One hypothesis regarding their relationship is the maternal modulation hypothesis, which predicts that the same neonatal stimulation may have different effects depending on the maternal context. Here we tested this hypothesis using physical development, indexed by body weight, as an endpoint and found that, among offspring of mothers with a high initial swim-stress-induced corticosterone (CORT) response, neonatal novelty exposure induced an enhancement in early growth, and among offspring with mothers of a low initial CORT response, the same neonatal stimulation induced an impairment. At an older age, a novelty-induced increase in body weight was also found among offspring of mothers with high postnatal care reliability and a novelty-induced reduction found among offspring of mothers with low care reliability. These results support a maternal modulation of early stimulation effects on physical development and demonstrate that the maternal influence originates from multiple instead of any singular sources. These results (i) significantly extend the findings of maternal modulation from the domain of cognitive development to the domain of physical development; (ii) offer a unifying explanation for a previously inconsistent literature regarding early stimulation effects on body weight; and (iii) highlight the notion that the early experience effect involves no causal primacy but higher order interactions among the initial triggering events and subsequent events involving a multitude of maternal and nonmaternal influences.maternal stress | maternal care | maternal mediation | developmental plasticity | growth enhancement I t seems self-evident that, for a developing infant, both the maternal and nonmaternal environment affects his or her development. Over the past century, ample evidence from both human and animal early-experience studies has confirmed this intuition (1-3). What remains challenging is how these two environmental sources interact to jointly influence offspring development (4-7). Emerging from the rodent developmental literature are two alternative views regarding the relation between maternal and nonmaternal influences. The "maternal mediation view" assumes that nonmaternal environment exerts its effect indirectly through its effect on the mother, who in turn affects the offspring (e.g., see refs. 8 and 9). When taken to the extreme, this view would directly contradict the stress-activation hypothesis (10, 11), which states that the nonmaternal environment has a direct effect on the offspring by activating some aspects of the offspring's hypothalamic-pituitary-adrenal (HPA) function. In contrast, the "maternal modulation view" incorporates the stressactivation hypothesis and assumes that the nonmaternal environment first activates aspects of the HPA axis, and the mother then modulates this effect (12). According to the latter view, the individual mother sets distinct conte...