 In their extreme forms, which treat the impairment or environment (respectively) as the sole cause of personal and social limitation, the medical and social models have few, if any, defenders. Rather, they mark the limits of possible relationships between impairment and limitation. Almost all writers on disability acknowledge some role for both the impairment and the environment in causing limitations; the disagreement largely concerns the assessment of their comparative contribution, and their interaction. Most scholars who embrace some version of the social model acknowledge that impairments—generally but not universally understood as deviations from species-relative statistical norms—can be sources of discomfort and limitation even in the absence of disadvantaging social practices (e.g., Shakespeare 2006, Thomas 2004). These scholars would argue, however, that such adverse effects are far less damaging than social exclusion, are greatly magnified by hostile environments, and could be significantly reduced by more inclusive environments. The medical model is less often explicitly defended than unreflectively adopted—by health care professionals, bioethicists, and philosophers who ignore or underestimate the contribution of social and other environmental factors to the limitations faced by people with disabilities. Even among these groups, however, there is a growing awareness of the environmental contribution to disability and a partial embrace of the social model (Cureton and Brownlee 2009).