Enter the Neuro-Economists: Why Do Investors Do What They Do? - New York Times:
"For instance, when humans are in a 'positive arousal state,' they think about prospective benefits and enjoy the feeling of risk. All of us are familiar with the giddy excitement that accompanies a triumph. Camelia Kuhnen and Brian Knutson, two researchers at Stanford University, have found that people are more likely to take a foolish risk when their brains show this kind of activation.
But when people think about costs, they use different brain modules and become more anxious. They play it too safe, at least in the laboratory. Furthermore, people are especially afraid of ambiguous risks with unknown odds. This may help explain why so many investors are reluctant to seek out foreign stock markets, even when they could diversify their portfolios at low cost.