• Which fund managers will outperform.
• Which way the economy is headed.
• What’s next for interest rates and share prices.
• Whether the overall stock market is overvalued or not.
• Which individual stocks will beat the market.
• Which stock market sectors and national stock markets will fare best.

Fortunately, this “knowledge” never greatly influenced my investment strategy. Still, I have little doubt that my portfolio’s performance would have been better if I hadn’t imagined that I knew these things—and I certainly wouldn’t have wasted so much time and mental energy.

Why do we think we know such things? It’s partly because Wall Street and the financial media talk endlessly about these issues. The financial media needs to fill airtime, websites and printed pages. Meanwhile, Wall Street wants to convince you that you know something about the future, so you actively manage your portfolio and thereby fatten the Street’s coffers.

But not all the blame belongs to others. Our belief that we have knowledge also partly stems from the way we’re wired. That wiring leaves us vulnerable to a host of behavioral mistakes, including extrapolation, overconfidence and recency bias, which together conspire to convince us that we know what the future will bring.

The problem: When we think we know something, we’re inclined to act upon that knowledge. In the financial markets, action almost always triggers investment costs and perhaps big tax bills. If we mess with our basic mix of stocks and conservative investments, we may miss a big market move—and any time we opt to reduce our overall stock exposure, we also lower our portfolio’s expected long-run return. And if our purported knowledge causes us to make narrow investment bets, we risk a permanent loss of capital, as we bet on stocks and market sectors that could potentially plunge—and never bounce back.

Even if we put our hands on our heart and we swear we aren’t inclined to forecast, predictions often creep into our behavior. We hold off investing because we sense share prices could fall. We tilt toward U.S. stocks because we think that they’ll always outperform foreign markets. Instead of prudently diversifying, we hang on to the employer shares we’re granted, because we can see that the company is prospering and we believe the stock price doesn’t fully reflect that.

What if we thought harder about such issues? No matter how much we analyze individual stocks, different market sectors and the overall market, there’s no evidence we’ll come up with a better forecast. Instead, our best bet is to not forecast. We need to unknow these things that we think we know, and instead focus on facets of investing where we have some control and where we truly can add value. We’re talking here about the amount of portfolio risk we take, the investment costs we incur and the taxes we pay.

There are also other areas of our financial life where hard work and more thought can pay handsome dividends. We can substantially improve our financial life by figuring out which debts to pay off first, what sort of home it makes most sense to buy, when to claim Social Security, what insurance we need and what estate planning steps we ought to take. We can also improve our life by spending more thoughtfully and saving more diligently—and by putting in the hard work needed to change our own damaging financial behavior.

To be sure, none of this has the seductive pleasure of making forecasts and imagining we’ll be proven right. But over a lifetime of investing, that pleasure, alas, almost always carries a steep price tag—and that’s one thing we all need to know.

This column originally appeared on Humble Dollar. It has been republished with permission.