For indexes, specifically SPX and RUT, does the smile get steeper or flatter during times of high volatility? For example, last week's high volatility seemed to flatten the smile, i.e. a short dated option 50 points off spot had a similar implied vol to an ATM option where as three weeks ago it would have had a higher implied vol. Was that just last week, was I imagining things, or is that generally the case? It seems like a simple question but I can't really find any literature examining it with any rigor, anyone have any references for me? Thanks in advance.