Spanier, who holds a doctorate in sociology from Northwestern University, did his thesis on wife swapping in the 1970s and wrote on this topic for the Journal of Sex Research in February, 1974. In his article, “Co-marital mate-sharing and family stability,” he wrote, “…the evidence available to date offers no evidence to suggest that mate-swapping is harmful to marital or family stability.” OMG! Is that true? I actually liked that guy.
Look at how slippery the part in bold is. Let me paraphrase:
Studies haven't been done establishing this to be true, so one is tacitly encouraged to believe it is false.
Scumbag.
the evidence available to date offers no evidence I had read the article, but didn't notice this linguistic monstrosity until you quoted it. My own feeling is that grammatical and syntactic oddities generally indicate that something else weird is going on.