Harlow’s experiments on the Nature of Love were a series of landmark studies on maternal deprivation in young rhesus monkeys that were conducted by Dr Harry Harlow at the University of Wisconsin-Madison, starting in the 1950s.  His findings had powerful implications for child-rearing, as they shed light on the nature of love, particularly the relationship formed between caregiver and infant.

His findings were pivotal in the evolving science of attachment and loss, and showed that “nurture” has a far greater impact on healthy human development than “nature”, and that emotional insecurity experienced during critical periods of development in early life can have devastating, permanent consequences in the later stages.  Despite using highly controversial methods in his experiments, Dr Harlow’s work was universally treated as a breakthrough in child psychology.     

In a lab at the University of Wisconsin-Madison, Harlow separated young rhesus monkeys from their mothers a few hours after birth and gave them a choice of two inanimate surrogate monkey mothers – one made out of wire but covered with soft terry cloth and the other made completely out of hard wire mesh. The wire mother additionally had a milk dispenser to provide nourishment. Surprisingly, the young monkeys clung to the soft, terry cloth mother the majority of the time and used the wire mother only for nourishment.

Next, the monkeys were separated into two groups: half of the monkeys had access to the terry cloth mother, while the other half only to the hard wire mother. Despite both groups showing similar feeding patterns and rates of growth over time, macaques that lacked this “contact comfort” associated with the terry cloth experienced greater psychological stress, which manifested in digestive problems. Cuddling with the terry cloth surrogate enabled the monkeys to develop emotional attachment and provided them with feelings of reassurance. 

Finally, Harlow placed the monkeys in dark isolation chambers he referred to as “the pit of despair” for various time periods (the longest was 15 years), that precluded contact with their peers. If the period of isolation lasted more than three months, the monkeys became severely traumatised and exhibited behaviours such as self-mutilation, emotional anorexia, and blank staring. After they were removed from the chambers, exposure to monkeys who had been reared normally was often insufficient in reversing this devastating effect (as a side note, his experiment also became a model for studying human depression). After placing the monkeys in isolation, Harlow attempted to use various forms of therapy to rehabilitate them to normality. However, his efforts were met with limited success.  

His experiments were revolutionary at the time, as they explored the influence of the caregiver-child relationship on mental health during the early stages of life. His work highlighted the dangers of institutional childcare, while favouring continuity of care associated with the early adoption of infants. His work, which would undoubtedly be considered highly unethical today, showed that love cannot be reduced to the simple satisfaction of physiological urges, such as hunger, and that emotional attachment is critical to healthy human psychological development.