Seasonal-Affected-Disorder is not just for investment bankers who never see the light of day. As is clear from the chart below, the August/September seasonal variation is quite dramatic for US equities. With only a 44% chance of advance in September (compared to ~60% average for the other months) and an average drop of -0.62% for thelast 60 years, the current slide in the S&P back to the June FOMC cliff-edge and a rising expectation for a Sept-Taper, it seems history may well repeat.
Seasonals are not helping...
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as the S&P tests back to the June FOMC cliff-edge...
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Charts: BofAML and Bloomberg