How does household portfolio diversification vary with financial sophistication and advice?

Economic theory suggests that households should invest their financial wealth in a combination of cash and a well-diversified equity portfolio. Yet, many households' equity investments are strongly concentrated in a few assets. Attempts to explain this discrepancy have included low levels of cognitive skills and/or financial knowledge; and poor or misguided financial advice. In order to investigate these claims empirically, I construct detailed portfolios for the respondents to a Dutch household survey. The data allow me to estimate the portfolios' risk-return properties without resorting to assumptions about characteristics of specific asset classes. Controlling for a large number of covariates, my results show that the combination of low numerical-financial skills and not seeking advice from other persons is strongly associated with the largest losses from underdiversification, whereas financial knowledge does not seem to have much of an effect.