Tesla TSLA shares -11.41% are in free-fall, on the verge of their worst year ever. Investors are selling shares because of concerns about demand for electric cars, Tesla’s own share schedule and some decisions by CEO Elon Musk. There is another reason for the stock drop in December: Taxes.

Investors can always sell some of their losing stock at the end of any year to collect a capital loss, which can then be used to offset the capital gains tax from the sale of winning stock in the same year.

This collection of tax losses can have a real impact. Researchers have studied year-end stock price movements over the years and have concluded that tax planning can partially explain why stocks make some of the changes they do in December and January.