Twin studies compare the behavior of monozygotic twins, who share a genetic code, and dizygotic twins, who are no more genetically similar than ordinary siblings.

However, monozygotic and dizygotic adult twins have equally similar microbiota, suggesting environment rather than genetics may drive familial similarities.

“Monozygotic is what people mean,” Boklage continued, insisting on the terms “monozygotic” and “dizygotic” when referring to twins.