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, many people are fascinated to see two individuals who appear so much alike. In addition to turning heads 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, meaning that they result from the same single fertilized egg, or zygote. In monozygotic twins, the zygote splits and leads to the development of two separate individuals. Monozygotic twins are distinct 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 features 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 genomes provide an intriguing opportunity for biologists and psychologists to explore the age-old nature-versus-nurture question: What characteristics of human beings are dictated by our DNA, and where does the external 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 and mannerisms, hobbies, careers, and lifestyles, despite having lived completely divergent lives since early infancy. Findings from such twin studies suggest that genes have considerable impact on both physical and behavioral qualities, largely outweighing the effects of environment and experience.Other studies have examined the concurrence of disease and aging between identical twins and fraternal twins. Scientists have uncovered a higher percentage of disease incidence in identical twins than in fraternal twins for many conditions, including cancer, Alzheimer’s disease, and schizophrenia. Higher rates of these conditions amongst monozygotic twins has lead to the conclusion that human health and longevity have strong genetic underpinnings.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 status of families who adopt and raise separated twins is very similar. Because of the parallels 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 even shown that identical twins raised in the same home can exhibit vast differences in their personalities; likewise, one identical twin may develop a disease linked to genetics while the other remains unaffected. While the science of twins has not closed the book on the nature-versus-nature debate, identical twins have helped researchers gain new knowledge about human genetics and outcomes. Many scientists now concur that our human qualities result from both genetics and environmental experience, and numerous identical twins have contributed to this compelling idea.