Thomas Reid argued that the human default trust in testimony is a gift of nature, which is sustained by two principles that "tally with each other", the propensity to speak the truth, and the tendency to trust what others tell us. Interestingly, he observed an embodied aspect of this trust:
It is the intention of nature, that we should be carried in arms before we are able to walk upon our legs; and it is likewise the intention of nature, that our belief should be guided by the authority and reason of others, before it can be guided by our own reason. The weakness of the infant, and the natural affection of the mother, plainly indicate the former; and the natural credulity of youth, and authority of age, as plainly indicate the latter. The infant, by proper nursing and care, acquires strength to walk without support (1764, Inquiry into the Human Mind, chapt VI, Of Seeing)
Reid's observations point to an intriguing possibility: to what extent is social cognition, such as trust in testimony, influenced by our bodily position, in particular the position we have as helpless infants? The Japanese primatologist Tetsuro Matsuzawa has argued that the supine position (that is, position on the back) of human newborns, has been a decisive factor in the evolution of human social cognition.
Humans and chimpanzees differ quite markedly in how much they trust others. For instance, although both chimpanzees and humans imitate, human children are more prone to overimitation than juvenile chimps, the children, but not the chimps, indiscriminately follow actions by an adult that are reduntant in obtaining a desired result (see e.g., here).
To my knowledge, Matsuzawa is unfamiliar with Reid; he bases his claims on detailed observations of chimpanzee mothers and their infants in his lab. Primates in the Kyoto Primate Institute are housed in family groups, often composed of mothers with infants, and maintain close proximity with human caregivers (see photo).
This is in contrast to primates from most western groups, which are usually housed in a zoo context, in groups of individuals of the same age, and there is less contact with human caregivers. The Kyoto lab thus provides researchers with lots of hours of observation of chimpanzee infant-mother pairs.
Matsuzawa argues that the supine position of human infants (as opposed to the non-supine position of other primates), provides an embodied way to facilitate eyecontact, imitation, trust in caregivers. Human infants have a large head, weak neck muscles, and are helpless at birth. Humans as a result frequently hold their infants, facing them, on their backs in a cradling position.
By contrast, newborn chimps are far more mobile and self-reliant at birth (they have the mobility and agility of a one-year-old child), and can clammer on the backs of their mothers. This position encourages early exploratory behavior and independence. Indeed, Matuzawa describes that chimpanzees cannot assume the cradling position that Reid thought so conducive to the trust young children place in their caregivers:
Chimpanzee infants, as well as orangutan infants, cannot assume a stable supine posture when they are laid on their back. They slowly lift up one arm and the contralateral leg, then a few seconds later, they switch both arms and legs and lift those on the opposite side. The alternation in limb movement suggests that this is not a stable posture for them—ape infants need to cling to the mother (S5)
Human infants are not only frequently placed in a supine position, they are also physically separate from their mother from day 1:
this, together with a stable supine posture, leads to a great deal of face-to- face communication, including looking into each other’s eyes, smiling, waving, and vocal exchange. Chimpanzee infants never cry at night: they have no need to do so as they are constantly embraced by the mother. Human infants, on the other hand, are often physically separated from the mother during both day and night, and receive a lot of allomothering behavior from family members (S9).
I find it intriguing that posture, by itself, could have such a large effect on cognitive development. If it is true, it has important implications for epistemology and embodied/situated cognition views, indicating that our philosophical views on testimony need not just take into account a fairly disembodied view of the interlocutor and hearer as exchanging information, but also an embodied view of the physical context in which trust in caregivers and, later, others develops. Reid was surely on the right track to trace our extensive reliance to testimony developmentally to the infant - caregiver (mother) relationship. As several studies have shown, young children prefer the testimony of familiar caregivers to that of strangers.
Such a preference may make evolutionary sense in that familiar caregivers are more likely to have your best interests at heart (as Sterelny recently argued, the world is not a uniformously epistemically hostile place, but contains pockets of epistemic benign places, for instance, parents and other caregivers), but it also makes developmental sense, in that the supine position promotes extensive eyecontact and plenty of early opportunities to learn through verbal and non-verbal testimony from close caregivers.