I read this week that 1% of young children are being diagnosed with some form of autism. There's some skepticism that this is a Type I statistical error, but the frequency of this disorder has clearly been increasing over time. Such increases must be occurring for a reason, so I can understand the alarm and paranoia that many parents have about everything from flu vaccines to lead in the water.
One argument that I've heard that I don't buy yet is that this is a recent trend in human evolution. For this to be evolution these changes would have to occur do to some common inherited trait of the parents of autistic children that specifically increased their rate of reproductive. At the moment I'm aware of no correlated pattern, leading me to suspect an environmental cause.
Of course, this increase in autism could still become part of future human evolution. The significant number of functional adults with autism mean that any inheritable tendencies for autism are likely to persist for generations. However, given the impairment of social interaction that defines autism, I find it unlikely it will ever become a majority trait in humanity.