There are few issues as hotly contested â?? and as poorly understood â?? as the question of what makes a person gay or straight. It's not only a political, social, and religious question but also a scientific question, one that might someday have an actual, provable answer. The handful of scientists who work in this under-funded and politically charged field will tell you: That answer is a long way off. But as Lesley Stahl reports, their efforts are already yielding tantalizing clues. One focus of their research is twins.