For investors, fear can be the portfolio killer. It can spur them to sell assets at the wrong time—or adopt a portfolio allocation that may be too conservative for their goals. Fiona Greig, global head of investor research and policy for Vanguard's Investment Strategy Group, spoke with Barron's about the relationship between investors' emotions and their financial behavior. She explained how to overcome fears of a market crash, the importance of first experiences, and why having $2,000 in emergency savings ...