Telecoms emerges as rare September outperformer

September's weakness was even more pronounced during U.S. midterm election years. None of the 11 S&P 500 sectors posted a cumulative gain in September during midterm years since 1926, according to Kaeppel's analysis. Real estate, financials, technology, materials and industrials were again among the weakest-performing groups.

"There can be no debate that the stock market - especially certain sectors - have tended to perform poorly during September. And in the long run, investors might have saved themselves some money (and angst) by lightening up, hedging, or steering clear in some way. However, the conundrum is that what is up for debate is how the stock market as a whole, or any sector or group of sectors, will perform during September this year," Kaeppel said.

The telecommunications sector, now classified as communication services .SPLRCL, has been a rare bright spot in an otherwise weak September track record for the S&P 500 .SPX, according to research by SentimenTrader's Jay Kaeppel.

Communication services posted a cumulative gain of 2.9% across the past 99 Septembers, making it the only S&P 500 sector to generate a net positive return during the month, according to Kaeppel's analysis of the index's 11 sectors dating back to 1927.

Real estate .SPLRCR, technology .SPLRCT, materials .SPLRCM, financials .SPSY and industrials .SPLRCI were the worst September performers. Real estate posted a cumulative loss of 85.1% over the period, followed by technology at 82.6%, materials at 79.7%, financials at 76.5% and industrials at 69.0%.