The ability to establish relationships with congeners is a prerequisite for the welfare of social species. However, some of them, such as the domestic horse (Equus caballus), are kept in social isolation, which is a stressful situation for the animal. The negative consequences of individual housing on the welfare and health of equids are now widely known and may also have effects on emotions and cognition. The aim of this study was to assess the impact of socialisation on the emotional state of 20 horses housed in individual stalls. The population was separated into two conditions: horses with no opportunity for social interaction (isolated condition) and horses with the opportunity to interact with a conspecific temporarily (social condition). The latter met in pairs for 1 hour/day in a device allowing social interactions during 4 months. After this period, all the horses passed a judgment bias test (JBT). The assumption of this test is that the living conditions of an animal can change the way it perceives its environment and lead to a judgement bias. The JBT estimates whether the horse perceives the information in its environment as positive, i.e in an optimistic way, or negative, i.e. in a pessimistic way. In term of methods, the subject is first trained to learn a positive and a negative position for a bucket. Then the bucket is placed in ambiguous positions, between the positive and the negative position without prior learning. The approach (GO response) and the latency to reach the bucket are recorded. Horses in the social condition approached the near negative ambiguous position more often (mean % of GO responses: 56% vs 15%, GLMM, p=0.03) and faster than horses in the isolated condition (mean latency = 94.62 vs 151.61 sec, Cox model, p=0.04). These results suggest a more positive perception of the environment for horses that were allowed to have social interactions compared to those housed in social isolation. Therefore, it appears that maintaining daily social contacts has a positive effect on the affective state and improves the overall welfare of the horses.