 Beyond a shared concern with the social causation of disability, different versions of the social model emphasize different features of the exclusionary structures and practices (see SEP entry on  disability: definitions, models, experience).  The minority group model regards people with impairments as a stigmatized minority group. It holds that the main reason people with disabilities encounter special hardships is that they face discrimination akin to that faced by racial or ethnic minorities (Hahn 1987, Oliver 1990). The human variation model holds that many of the challenges faced by disabled people do not result from deliberate exclusion, but from a mismatch between their characteristics and the physical and social environment (Scotch and Schriner 1997). These two versions of the social model differ mainly in emphasis. The discrimination stressed by the minority group model generally leads to, and is expressed in, the societal failure to accommodate people with stigmatized differences. Yet the failure to fully accommodate people with various differences, from extreme height to intellectual impairment, does not necessarily arise from stigma. But often, disparities in access that were initially caused by resource or technological limits are maintained by stigma. For example, an employer’s initial purchase of communications technology inaccessible to blind or deaf employees may be explained by the fact that accessible technology had not been developed at the time of purchase. But those historic facts would not justify the employer’s refusal to include modestly priced accessibility features when purchasing upgrades a decade later, for example.