Last month's New England Journal of Medicine has a great lesson about how we should think about risk factors. It's been long known that homocysteine is a risk factor for heart disease. That means that, when looking at large numbers of people, those with high levels of homocysteine have more heart attacks on average than those with low levels. Too frequently, we confuse a risk factor with a cause, and we jump to the conclusion that homocysteine causes heart attack. A risk factor, howeve...