After reaching a fresh high on September 2, the SPX and NDX executed an abrupt turnaround that frankly came a bit out of nowhere. That’s not a particular surprise, of course, because volatility usually rises as stock markets drop. But it is still worthwhile to disentangle what is driving the abrupt increase in volatility. Although there have been some high-profile reversals in much-trafficked technology (and pseudo-technology) stocks, in aggregate short-term single-stock volatility hasn’t actually moved that much. Granted, 10-day realized vol has risen -- from 27% on September 2 to 34% as of Thursday’s close -- but some perspective is in order. That 34% reading is still below where it was in mid-late July, even as stocks were rallying during earnings season. That the short term volatility of the index has converged with single stock volatility tells us that correlations have risen sharply. #stocktalk