Shari Finkelstein

Marcel is an adorable one-inch-tall shell who ekes out a colorful existence with his grandmother Connie and their pet lint, Alan. Once part of a sprawling community of shells, they now live alone as the sole survivors of a mysterious tragedy. When a documentarian discovers them amongst the clutter of his Airbnb, his resulting short film brings Marcel millions of passionate fans, as well as unprecedented dangers and a new hope at finding his long-lost family.

There are few issues as hotly contested — and as poorly understood — 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.