ABSTRACT: As with most primates, optimal development in infancy proceeds with a high degree of intimacy and interaction between infants and caretakers. Human infants are less developed at birth than most primates because of selection for a greater percentage of brain growth to take place after birth than in utero. Reasons for delayed development of brain include the high degree of intimacy between maternal and fetal systems because of the hemochorial placenta; the upper limits to intrauterine brain development imposed by the maternal pelvis; and the advantage of having the brain develop in the more challenging environment outside the uterus. The impact of delayed development on the caretaker is reviewed, as are the effects of the quality of the neonatal environment on subsequent development of visual, locomotor, linguistic, and manipulative skills.
Cross-species comparisons have been used extensively in research on human infancy, most notably the work of the Harlows and their colleagues (Harlow, 1958; Harlow and Harlow, 1965; Harlow, Harlow, and Hansen, 1963) and Hinde and his colleagues (Hinde, 1974; Hinde and Simpson, 1975; Hinde and Spencer-Booth, 1971). Nonhuman primates are often cited as appropriate animal models for human development because of their close genetic and evolutionary relationship to our species. For the early neonatal period, however, nonhuman primates may not be suitable because of differences in the state of development of human infants relative to other higher primates. Primates typically give birth to precocial young which are able, within minutes of birth, to crawl along their mothers' abdomens and cling to their fur unaided. Human infants are far more altricial in some systems and lack the motor skills to cling to their mothers, even if suitable body hair and a grasping hind limb were available. This degree of altriciality sets limits and potentials for the developing human infant during the first few months of postnatal life. This paper will review the possible evolutionary events leading to selection for relative helplessness in the human infant and offer observations on the consequences of this state for perinatal behavior and development.
COMPARISON OF HUMAN AND NONHUMAN NEONATES AT BIRTH
For the first few months of postnatal life, the human infant shares the growth pattern of other primate fetuses, not of other primate infants. Ashley Montagu called attention to this phenomenon in a 1961 article in which he referred to the first nine months of postnatal life for the human infant as "exterogestation" to distinguish them from the nine months from conception to birth, or "uterogestation." In his argument birth is merely the transition from one gestational stage to the other; it is not the end of gestation as is traditionally believed. Further, he argues that for the human infant, birth is analogous to that of marsupials: Clearly gestation is not completed for marsupials at the time they leave the uterus to continue development in the pouch.
Earlier, Portmann (1941, 1945, cited in Gould, 1977) had proposed that the human gestation period was actually 21 months and that we continued part of our development outside the womb, as "extra-embryonic embryos." The most obvious system that develops "embryologically" for the first few months of postnatal life is the neonatal brain. At birth, the human brain is about 25 per cent of adult size, measuring about 350 cm^sup 3^ (21 in^sup 3^). By one year it will have more than doubled in size to approximately 800 cm^sup 3^, and it is not until age three that the brain reaches 75 per cent of what its adult size will be. The rate of growth is much more typical of fetal development than of early neonatal rates when compared to other species. Rhesus macaque infants are born with about 70 per cent of adult size and chimpanzees with about 40 per cent. These species double their brain size by the end of the first year of postnatal life.
Other systems are also immature at birth. …