To give my tentative thesis the weight of a generalization, I went digging through the KU Libraries' PsycINFO database for adult-outcome studies of autistics and Aspies. I lucked out in that my first two hits were literature reviews: Gena Barnhill's 2007 "Outcomes in Adults with Asperger Syndrome" and Patricia Howlin's 2000 "Outcome in Adult Life of More Able Individuals with Autism or Asperger Syndrome." Both articles identified trends among autistic adults to be either unemployed or employed far below their expected skill level, and to have difficulty holding onto jobs. Most of the studied autistics who worked, worked in specially supported environments, rather than in the (competitive) job market. Few lived independently, with most living either with parents or in residential care. The studies that tracked variables correlating with greater success in getting and keeping a job tended to agree that higher IQ (one study narrowed that down to verbal IQ) and more extensive social-support networks were the two greatest predictors of such success.
Two Swedish studies showed higher rates than the other (American and UK) studies of independent living, which the authors of one of the studies (Engstrom et al., 2003) attribute to the greater social supports available in Sweden, where there is generous government-funded healthcare. Even in these studies, though, most of the subjects were unemployed.
Leo Kanner and Hans Asperger, the two simultaneous "discoverers" of autism, both tracked the outcomes of their former patients: Kanner found that, of 96 people, 11 were employed and one at college. Seven owned their own homes; all the others lived either with their parents or in institutions. Asperger's patients had a much wider range of outcomes; he remarked that their special interests often drove them to become experts in that field, which "eventually led to [their] social integration," most often in academia.
Barnhill and Howlin both tied the mostly-dismal job prospects (and maddening job-hunting process!) autistic people face to our elevated rates of anxiety and depression.
Despite the potential to work, few persons with AS are in regular employment, and those who are employed find their employment levels disappointing and their occupational status low. Many times jobs end prematurely, often leading to low self-esteem and depression (Goode, Rutter and Howlin, 1994).
Although high-functioning people with autism or Asperger syndrome may succeed well as adults, such achievements rarely come easily. Few specialist support systems exist and most individuals have to rely heavily on the support of their families in finding jobs or accommodation. ... Above all, there may be constant pressure to 'fit in' with the demands of a society that fails to understand their needs or difficulties. Inability to meet these demands may lead to stress and anxiety and even psychiatric breakdown.
I will point out here that I do not entirely approve of the use of "high-functioning" and "low-functioning" to describe autistic people. I do not think those labels are terribly descriptive, as they refer just to IQ, and autistic people don't get the same kinds of results on IQ tests that NTs do. We do better on some tests than others, and vary wildly within a test (say, the Weschler), having peaks in one domain and valleys in another, when NTs taking the same test will have a constant score across domains. Also, many of us are completely nonverbal, which makes accurate intelligence testing difficult.
That aside, I think Howlin's point about autistics being effectively marooned within our families is a good one. I think it's a particular failing of American culture that we have eradicated all forms of social support except the nuclear family. Widespread mobility, suburbanization and car culture have broken up extended families and neighborhoods, and the fear of creeping communism has kept us from developing any kind of state-run social support systems!