On the biological side, modern neurology says the number of neurons in the human brain reaches maximum value before birth at about 13 billions. After this point, the increase in weight consists of increased numbers of fibers, increased connections, increased size of elements, and increased efficiency and selectivity of transmission. Thus the increase in weight of the human brain from about 400 to 1400 grams seems to be devoted to improving its internal (as well as external) communication, storage, and computation networks. As I have stated elsewhere (_Man and Dolphin_), it is my impression that there exist critical threshold values in the brain’s growth pattern at which certain kinds of performance become possible. Complex speech acquisition seems related to brain weights of 800 to 1000 grams, but no smaller. This assumes, of course, numbers of neurons (10¹⁰) and numbers of connections and opportunities for learning and time to learn commonly found with humans. The critical psychological factors in speech acquisition are slowly being dug out and described.[21][22] Among these the most important seem to be a continuous background of presentations to the child in rewarding circumstances of speech and its close relations to objects, actions, satisfaction of needs, and persons. Imitation of one’s use of facial and vocal apparatus appears spontaneously in the happy child. The virtuosity of the child as a mimic is truly astonishing. I am also impressed by evidence for what I call the “transactional drive.” A bright child seems to seek and respond best to those persons who respond in kind, back and forth in exchanges of sounds and linked actions. For example, if one starts such a transaction with a child of 22 months with a loud word, if he is ready, he may return his version of the word or a slight variant; if one replies with another variant the child replies with still a third, or even suddenly with a new word, and so on back and forth in a transactional vocal dance. Or one may reply to a child who invites such an exchange to begin. Such exchanges seem to function as rewards of themselves, and hence the name, “transactional drive.” This phenomenon is more than mere mechanical slavish mimicry. It seems to aid in perfecting pronunciation, increases vocabulary, increases the bonds with other persons, serves to substitute the “consensus-dictionary” words for the private baby words, and is thus essential to learning a language of one’s own species. It is thus that the child “becomes human.” As the child ages and grows, the exchanges lengthen, and the time during which each member of the dyad is quiet while the other speaks becomes longer, until finally for a half hour or so, I am lecturing and you are at least quiet, if not listening.