A small number of (mega-cap) names have done the bulk of the heavy lifting for market-cap-weighted S&P 500 performance this year, but that does not steal from the fact that breadth has been solid. As shown below, the S&P 500's 10-day advance-decline line—which essentially measures the ten-day average of the daily net percentage of stocks in the index trading higher/lower—has shown positive readings 57.9% of the time in 2023 and is right in line with the annual average dating back to 1990.

Within this year's respectable breadth readings has been a particularly strong run over the past couple of months which we first highlighted in 

As shown below, the 10-day advance-decline (A/D) line has pretty consistently sat at least one standard deviation above its historical average throughout November and December. In other words, the past couple of months have consistently seen historically large swathes of the market trading higher.

Looking at the past 50 trading days, two-thirds of the time have seen the 10-day A/D line sit in overbought territory. Of all rolling 50-day periods, there have only been two other times when breadth by this measure has been overbought on such a consistent basis. The most recent of these was in early 2019 when it was overbought 70% of the time. Before that, you'd need to go back to 2010 to find a reading as high.

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