Keeping own-company stock, ala Enron. In general, unless your the owner of a company, you want as little company stock as possible. Otherwise your wealth and your income are tied to the same risk factor.
Investing in any single-company stock or bond. In general single-holdings are more subject to behavioral biases than others.
International or location Home bias: We all love our country/state/city. That doesn’t mean we should invest more there. In fact, we should invest less, so that when our economy tanks, our portfolio is a bit diversified away from it.
Disposition Effect – – holding onto losers, selling winners. It feels good to be right, and it hurts to admit you were wrong. Over the long run, this leads to a portfolio of losers.
Driving by looking through the rear-view mirror isn’t smart. It isn’t for investing either. All technical/chartist trading rules don’t stand up to statistical testing and aren’t robust. Nor do trends or anything else.
Don’t trade a lot – you lose out of fees. The institutions make money off you, but your expected gain is actually negative because of their costs.