In a large online experiment, we study how investors assess the relationship between their portfolio and the stock market. Participants either select a portfolio of stocks or are randomly assigned a portfolio from a U.S. stock market index. They state their portfolio return expectations conditional on different market outcomes, allowing us to calculate implied beliefs about portfolio beta. We find a general underestimation of beta, which is particularly strong for downside beta. This asymmetric assessment of dependence is amplified for participants who select a portfolio themselves instead of receiving a randomly assigned portfolio. They believe their portfolio goes up with the market but does not come down with it. Our findings reveal yet unknown patterns in beliefs about systematic risk, which shed light on the source of investor overconfidence.
JEL codes: G11, G12, G41.
Keywords: Return Expectations, Risk Expectations, Beta, Diversification, Overconfidence.