It is clear that puberty is brought on by the physical maturation of neurochemical systems underlying sexuality, with striking physical and psychological changes associated with the mature production of sex hormones. Harlow suggested that there is an analogous maturation of neurochemical systems underlying fear and anger that support the events typically occurring in the ecology of development. At birth, the infant show signs of being able to display, and presumably experience, feelings of affection, bonding, and love; and also pain and distress. However, there are few signs early on of displays of fear or anger. A human infant for example will accept strangers as well as familiar persons until about five months of age, when many begin to show signs of wariness: suspicious looks, turning away, whimpering and crying.  Signs of anger similarly do not appear until two-three months of age, and the toddler's capacity for anger often reaches a peak in the so-called "terrible twos," beginning about 18 months of age  or earlier,  characterized by pervasive negativity, saying "no," hitting, biting, and temper tantrums.

One of the observations that surprised Harlow in his studies of maternal deprivation in Rhesus monkeys was the high degree of fear and aggression displayed by animals isolated from other monkeys for the first year of life, even though they had no opportunities to learn fearful or aggressive behavior. This implied that the fearful and aggressive reactions are based upon innate mechanisms. Harlow's later observations suggested that the capacity for affection is present at birth, but there is little evidence of fear or anger. Signs of fear begin to be observed in the first few weeks, and become fully established by six months, and he suggested that the neurochemical mechanisms underlying fear mature at this point. However, aggression has not fully appeared. The six-month-old exhibits isolated components of the aggression response during play--biting, threat grimacing, submissive gestures--but it is not until the end of the first year that play occasionally becomes really violent and abusive. A dominance ordering emerges from these aggressive bouts, and fighting subsequently declines and is replaced by gestures of threat and submission. These become a cornerstone of adult monkey social organization, where intragroup violence is moderated by communication.

Thus, Harlow suggests that the neurochemical systems underlying affection, fear, and aggression develop in a fixed maturational sequence, but that the way the youngster learns to deal with and use these emotions--what I have termed emotional education--depends upon social experience. Normally, fear appears only after the infant has had plenty of time to become acquainted with, learn to trust, and form affectional bonds with other monkeys. Fear would not be functional during this period, the infant is protected by the mother and other adults: indeed a mature fear response  would tend to interfere with bonding. But fear does become essential after separation from the mother in the ecology of rough-and-tumble play. At this age, anger and rage would not be functional, as they could result in physical injury. Aggression does not fully appear until further social experience allows these dangerous emotion to be regulated and moderated by communication.

This model of emotional maturation is illustrated in the accompanying figure, which show an approximation of the initial high level of infant-mother contact being associated with low fear and anger; the drop in maternal contact and rise in peer contact after three months being associated with the maturation of fear; and the high level of peer contact after nine months associated with the maturation of anger. The model was supported in a series of deprivation studies by Deets and Harlow, presented at the convention of the American Association for the Advancement of Science in 1971 but not otherwise published. The overall picture that emerges is one of an interaction between biology and ecology, such that motivational-emotional potential becomes actualized in a social context that typically allows its regulation, management and control. A lack of the appropriate experience, at the appropriate time, can have devastating consequences on long-term social and emotional development.   

Portions taken from R. Buck, "Human Motivation and Emotion," 2nd Edition, Wiley 1988.