The laying hen industry aims to improve general disease resistance of laying hens. Genetic selection for increased natural antibody (NAb) levels is a promising strategy, because Nab levels are related to survival and are heritable in chickens. However, selection for increased NAb levels may increase feather pecking (FP), possibly through increased stress reactivity and reduced serotonergic activity. Therefore, the aim of this study was to determine the effects of divergent selection for NAb levels on feather damage, stress reactivity, and whole-blood serotonin levels. In total 389 White Leghorn hens in the high and low NAb lines were studied. From each line, 75 hens were chosen to measure feather damage and to test with a bucket test, open field (OF) test, social novel object (NO) test, tonic immobility (TI) test, and manual restraint (MR) test. Total levels of NAb binding Keyhole Limpet Hemocyanin (KLH), plasma corticosterone and whole-blood serotonin concentrations were also measured. High NAb hens showed significantly higher total levels of NAb binding KLH and had more feather damage than low NAb hens. In the bucket test, high NAb hens started to vocalise significantly later than low NAb hens. Furthermore, the number of high NAb hens that approached the NO in the home pen was significantly lower than in low NAb hens. There was no significant difference between the two lines in OF, TI and MR tests. Levels of plasma corticosterone and whole-blood serotonin were not significantly different between the two NAb lines. These results suggest that selection for high NAb levels results in more feather pecking and feather damage. In the behavioural tests, the high NAb birds seemed more fearful, indicated by longer freezing in the bucket and longer latency to approach the NO, but these differences were not supported by differences in stress physiology.