For captive animals, living in barren conditions leads to stereotypic behaviour that is hard to alleviate using environmental enrichment. This resistance to enrichment is often explained via mechanisms that decouple abnormal behaviour from current welfare, such as 'establishment': a hypothetical process whereby repetition increases behaviour's predictability and resistance to change. If such hypotheses are correct, then animals with enrichment-resistant stereotypic behaviour should still find enrichments rewarding. Alternatively, this behaviour could reflect a failure to improve welfare: plausible because age and chronic stress increase neophobia and anhedonia. If this hypothesis is correct, animals with enrichment-resistant stereotypic behaviour should value enrichments less than conspecifics. We tested these hypotheses using C57BL/6 mice, Mus musculus, aged 10e11 and 6e7 months, raised in barren laboratory cages. We observed their behaviour in both these and large enriched cages. Enrichment was more effective on the younger animals. However, contrary to ideas about establishment, the spontaneous predictability of stereotypic behaviour did not increase with age; nor was enrichment less effective on more predictable or timeconsuming forms. We assessed the reward value of enriched cages by allowing access via progressively weighted doors (maximum weight pushed corresponding to peak motivation). In older mice, those individuals whose stereotypic behaviour was least reduced by enrichment were also the least motivated to gain access to enrichment. This suggests that the welfare of middle-aged-animals, as well as their stereotypic behaviour, is harder to improve using environmental enrichment. Ó

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