Harry Harlow is best known for his groundbreaking experiments on attachment and social development using rhesus monkeys. He famously demonstrated that infant monkeys preferred a cloth surrogate mother that provided comfort over a wire mother that provided food, showing that contact comfort is more crucial for attachment than nourishment. His research also explored the devastating effects of maternal deprivation, social isolation, and early peer relationships. Harlow's work fundamentally changed the understanding of love and affection in both animals and humans, influencing child care practices and theories of emotional development, and he is considered one of the most influential figures in comparative and developmental psychology.

In Harlow's surrogate mother experiments, infant rhesus monkeys were separated from their biological mothers and raised with two inanimate surrogate mothers: one made of wire that dispensed milk, and another covered with soft terry cloth that did not provide food. Harlow observed that the monkeys overwhelmingly preferred the cloth surrogate, spending most of their time clinging to it and seeking it out when frightened, despite the wire mother being the source of nourishment. This finding demonstrated that emotional comfort and physical contact are more important than food in forming attachment bonds, challenging the then-dominant behaviorist view that attachment was based on feeding reinforcement.

Harlow's research profoundly affected attachment theory by providing empirical evidence that emotional bonds are not simply a byproduct of feeding. His findings supported John Bowlby's later formulation of attachment theory, which emphasized the evolutionary and biological basis of attachment for protection and survival. Harlow showed that contact comfort and security are primary drivers of attachment, influencing subsequent research on human infant attachment, maternal care, and the long-term consequences of early deprivation. His work also contributed to understanding the importance of responsive caregiving in institutions, leading to reforms in orphanages and childcare settings to provide more physical and emotional contact.

Harlow's experiments have faced significant ethical criticism for the severe distress and suffering inflicted on his monkey subjects. In isolation studies, monkeys reared in total isolation for months or years developed profound psychological disturbances, including stereotyped rocking, self-mutilation, and social incompetence. Critics argue that these experiments caused unnecessary pain and that the knowledge gained, while important, could have been obtained through less invasive methods. Harlow himself acknowledged the cruelty but defended the research as necessary for understanding human depression and attachment disorders. Today, his work is often cited as a classic example of the ethical tensions in animal research, and it influenced the development of stricter animal welfare guidelines.

Harlow discovered that peer relationships are critical for normal social development in primates. In experiments where infant monkeys were raised with only a surrogate mother but had access to age-mates, they developed relatively normal social skills, whereas those raised in isolation or with only a mother surrogate showed severe social deficits. He also created the 'peer therapy' paradigm, where socially isolated monkeys were gradually exposed to younger, normal monkeys, allowing them to develop social behaviors over time. These findings highlighted that early social interactions with peers are as essential as maternal care for learning species-typical behaviors, such as play, grooming, and aggression management, influencing our understanding of socialization in humans and animals.

Harry Harlow authored numerous influential papers and books. Among his most important works are 'The Nature of Love' (1958), which presented his surrogate mother findings and redefined the understanding of attachment, and 'Learning to Love' (1971), a book summarizing his research on affectional systems. He also published extensively on learning, motivation, and the effects of early experience in monkeys. His research papers, such as those on the 'love apparatus' and social deprivation, remain classic citations in developmental and comparative psychology. Harlow's integrative approach combined rigorous experimentation with theoretical insights, making his publications foundational texts for students and researchers in the field.

Harlow's work had a major impact on child development and mental health by highlighting the critical role of early emotional and social experiences for healthy psychological functioning. His findings contributed to the shift away from the behaviorist emphasis on feeding as the sole basis of attachment, promoting instead the importance of warmth, touch, and responsive caregiving. This influenced pediatric practices, hospital visiting policies, and the design of orphanages. Additionally, his research on isolation and depression contributed to the understanding of affective disorders and the development of animal models for clinical psychology, informing therapeutic interventions for attachment disorders and post-institutionalized children.

Harlow is considered controversial due to the ethical nature of his experiments, which inflicted profound psychological and physical distress on monkeys. His isolation studies, in particular, are seen as cruel and unnecessary by many, as they induced severe and lasting mental illness in his subjects. However, supporters argue that his research yielded essential knowledge about attachment, social development, and the effects of deprivation, which could not have been ethically studied in humans. This tension places Harlow at the center of debates about the balance between scientific progress and animal welfare. Despite the controversy, his empirical contributions are undeniable, and his work continues to be studied and critiqued in psychology and ethics courses.