Is the Stock Market’s ‘January Effect’ Real?

For decades, a popular theory has held that US stocks tend to rise more in January than in other months. The existence of this phenomenon, known as the January effect, once appeared to be undeniable as studies showed gains several times larger in January than in an average month. The effect was most pronounced among small-company stocks from 1940 to the mid-1970s. But it seemed to shrink through around 2000 and hasn’t been as reliable since.