OVER the last few weeks, as the stock market has reached new highs, my thoughts have turned to my 85-year-old mother.

“O.K. Mr. Smarty-Pants,” she often asks me, “what stock should I buy now?”

She first asked me this question when I was an undergraduate at Princeton, majoring in economics. She asked again when I was a graduate student at M.I.T., earning a Ph.D. in economics. And she has asked it regularly during the last three decades when I have been an economics professor at Harvard.

Unfortunately, she has never been happy with my answers, which are usually evasive. Nothing in the toolbox of economists makes us good stock pickers.

Yet we economists have written countless studies about the stock market. Here is a summary of what we know:

THE MARKET PROCESSES INFORMATION QUICKLY One prominent theory of the stock market — the efficient markets hypothesis — explains how answering my mother’s question would be a fool’s errand. If I knew anything good about a company, that news would be incorporated into the stock’s price before I had the chance to act on it. Unless you have extraordinary insight or inside information, you should presume that no stock is a better buy than any other.