Social deprivation in non-human primates causes severe and lasting psychological harm, and the evidence base for these effects provides the strongest scientific justification for social housing requirements.
The evidence that social deprivation harms primate welfare is among the most robust in all of comparative psychology. The Harlow experiments, while ethically troubling in their execution, established definitively that social contact is a biological need in primates, not merely a preference. Deprived primates develop stereotypies, self-injurious behavior, and permanent impairments in social competence that represent profound welfare failures. Adult single housing perpetuates similar though less severe harms. The EU requirement that single housing must be scientifically justified reflects this evidence base — the presumption is that social housing is the welfare standard and departures require justification. Implementing this presumption in practice requires institutional commitment to pair and group housing protocols that manage the technical challenges of social housing while realizing its welfare benefits.