Mystery of the ‘land of twins’

High atop a hill behind his family’s home, Derli Grimm knelt and took a sip from a thin black tube leading from a natural spring. Like so many in this farming town, populated almost entirely by German-speaking immigrants, Mr. Grimm, 19, believes that something in the water — a mysterious mineral, perhaps — is responsible for the town’s unusual concentration of twins. “It can’t all be explained by genetics,” said Mr. Grimm, himself a twin."