Stocks are trading in lock-step more than at any time since the 1987 crash, and the trend has some analysts concerned.

In recent weeks, stocks in the Standard & Poor's 500-stock index have shown an increasing tendency to move in the same direction at the same time. Last week, those stocks' tendency to move in the same direction as the index hit an extreme not seen since October 1987, according to research by investment group Birinyi Associates in Westport, Conn.

The...