It can be challenging to resist taking a second glance when you encounter a set of identical twins. While it is true that each human being is unique, there is something striking about seeing two individuals who appear so much alike. In addition to turning heads in public and inciting non-twins to wonder, “What would it be like?”, identical twins have offered valuable insights into human biology and development. Identical twins are known as monozygotic because they develop from the same fertilized egg, or zygote. In monozygotic twins, the zygote splits and leads to the development of two separate individuals. Monozygotic twins are different from dizygotic twins, known as fraternal twins, because dizygotic twins develop from two separate zygotes. As a result, identical twins typically exhibit more shared physical and behavioral traits than fraternal twins.In fact, identical twins share approximately 99.9% of their DNA, and differences in their personality and appearance can sometimes be imperceptible. Their nearly identical genetics provide a fascinating opportunity for biologists and psychologists to explore the age-old nature-versus-nurture question: What qualities and characteristics of human beings are dictated by our DNA, and where does the environment play a larger role? To study the influence of heredity compared to experience, scientists have studied identical twins adopted into different families at birth. In many instances, the separated twins were found to be “identical strangers”, sharing not only physical appearance but also personality traits, hobbies, careers, and lifestyles, despite having lived completely divergent lives since early infancy. Findings from such twin studies contribute to the theory that genes have considerable impact on both physical and behavioral outcomes, largely outweighing the effects of environment and experience.Despite these intriguing findings, scientists have long debated the strength of the claims that can be drawn from the results of twin studies. Often, the economic and social environment in which separated twins are adopted and raised is very similar. Because of the similarities in their upbringing, it is difficult to conclude with certainty that the shared traits of separated twins are due to genetics alone. Other studies have shown that even identical twins raised in the same home can exhibit vast differences in their personalities. While the science of twins has not closed the book on the nature-versus-nature debate, identical twins offer valuable insights into human biology and development. Many scientists now concur that our human experience results from both genetics and environmental experience, and many identical twins have contributed to this compelling idea.