Why does there seem to be a high proportion of paedophiles with autism or aspergers?

I've been watching a lot of the paedophile hunter sting videos lately and almost all of the people they catch either have autism, ADHD , or in some cases a more severe learning difficulty. 

Obviously not all autistic people are paedophiles , and not all paedophiles are autistic but why is there perhaps a disproportionately high number of child sex offenders who are on the autistic spectrum? 

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