There are multiple tax court rulings - go to Lexis/Nexis if you have a friend who has a subscription and the issues or both correlation and intent. Correlations have been really low and the ambiguity of the wording is intended to give the tax court ample leeway. The ruling everybody quotes in the industry - possibly just bullshit - is a correlation of 65% or above. So SPY and QQQ would trigger a wash sale. Tax courts are going to focus on intent and in tax court, you are guilty until proven innocent.