S&P 500 breaks 200-day moving average as ETFs erode signal
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- S&P 500 broke below its 200-day moving average last Thursday, a technical event that analysts once viewed as a strong bear-market warning but now question due to changing market dynamics.
- ETFs enabled easy, low-cost trading of broad market baskets, allowing widespread adoption of the 200-day moving average strategy, which ultimately eroded its effectiveness as a timing tool.
- Blake LeBaron, an economics professor at Brandeis University, found that the 200-day moving average strategy stopped working in the early 1990s when investors gained the ability to follow it cheaply and at scale.
- Historical investing strategies analyzed in academic research performed 58% worse after publication, according to a study titled 'Does Academic Research Destroy Stock Return Predictability?', showing that visibility kills edge.
- The 200-day moving average is no longer a reliable predictor of future market direction, with the article arguing its breakdown now carries no greater significance than any other technical level.
Why it matters: The erosion of the 200-day moving average’s predictive power changes how investors interpret market signals—traders relying on this indicator may face unexpected volatility, and the broader shift underscores how democratized strategies lose efficacy when widely adopted, altering risk assessment for millions in passive funds.
