For nearly a century, armies have been trying to select recruits for jobs they are best suited for. Oddly enough, the most difficult selection chore is for combat troops. For a long time, selection was restricted to insuring that the recruits were in good enough physical shape to march and fire their weapons. This changed during World War II, when selection of superior fighters became a life or death matter for commando type units. Thus was the "stress test" developed. Candidates, many of whom were already experienced infantrymen, were put in situations where they were tired, had little sleep, and then given difficult chores to accomplish in a short amount of time. Those who were best at this were selected, and the selections proved accurate. Similar selection and training methods continued after World War II. But now blood tests of these elite warriors, compared to regular infantrymen, shows that it's all in the blood. More specifically, the elite warriors have highly elevated levels the hormone neuropeptide-Y (or NPY). Not much is known about NPY, although it appears to be a natural relaxant. Produced in the brain and intestine, NPY is also involved with appetite control, heart function and the quality of ones sleep. If you are one of those people who just naturally have a lot of NPY, you tend to be cool under pressure and very capable of handling stress. We all know people like this. But if you want to be a successful commando, you really need NPY. This is because even commandos generate large quantities of the stress hormone cortisol when under pressure. Without NPY to handle the cortisol, you will be just as stressed out and exhausted as a normal person. It's not normal to have a lot of NPY. But a commando is more than some buff, aggressive guy with a lot of NPY. While there appears to be a lot of blood and brain chemistry issues going on here, there are also the more traditional items like motivation and physical fitness. But these have always been easier to measure. In the future, it looks like more blood tests will also be part of the selection process. Another spin off from this research will probably be an attempt to create drugs that will give all soldiers the same advantages. Troops have been taking ability enhancing drugs for nearly a century. Amphetamines have been the most popular, as sheer fatigue has long been a major, and often fatal, problem on the battlefield. But dealing with stress is nearly as big a problem. An effective anti-stress pill would be welcomed on the battlefield, for it would increase chances of survival in an often fatal occupation.