Facial expressions are common across mammals and are essential for social communication. In humans, a rich early social environment is important for the appropriate development of facial expression processing. Whether other animals are similarly reliant on social input for facial expression development, or have a more fixed system, is unknown. Here, we investigated how maternal care influences facial expression processing skills in rhesus macaques (). We conducted three experiments quantifying facial expression processing and examined performance in relation to historical maternal data and across age. Facial expression processing skill was predicted by positive social contact with the mother during infancy and increased with age until adulthood. Our findings provide the first evidence that early social input, specifically maternal care, enhances facial expression processing skills in non-human animals. This challenges the notion that facial expression processing systems are entirely hard-wired and innate and emphasizes the importance of flexibility and responsiveness to local conditions.