Assumptions can be dangerous to investors

Every day of our lives, we make assumptions. We assume that the people we encounter regularly will behave in the manner to which we are accustomed. We assume that if we take care of our cars, they will get us to where we want to go. In fact, we need to make assumptions to bring order to our world. But in some parts of our life — such as investing — assumptions can prove dangerous.

Of course, not all investment-related assumptions are bad. But here are a few that, at the least, may prove to be counter-productive:

• “Real estate will always increase in value.” Up until the 2008 financial crisis,...