January effect
Small stocks tend to pop in January, defying efficient markets.
What it means
A calendar anomaly in which stock returns, especially for small-capitalization firms, are abnormally high in early January. Leading explanations include tax-loss selling in December — investors dump losers to harvest tax deductions, depressing prices that then rebound — and window dressing by fund managers. As a seasonal, predictable pattern it contradicts the efficient-market view that prices already reflect all such information. It matters because, like other calendar effects, it has weakened since publication, illustrating how anomalies can shrink once arbitrageurs learn to trade them.
Examples
A portfolio of small, beaten-down stocks bought in late December historically delivered outsized gains in the first weeks of January.
A fund manager sells her worst holdings in late December so the loss lands on the tax return, adding to the selling pressure that sets up January's rebound.
Once the pattern was published and widely traded, the January premium on small stocks shrank — an anomaly that arbitrageurs have largely priced away.
First described in Rozeff & Kinney (1976); Keim (1983).