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Biological determinism
The belief in biological determinism was matched in the 20th century by a blank slate denial of any possible influence of genes on human behavior, leading to a long and heated debate about "nature and nurture". By the 21st century, many scientists had come to feel that the dichotomy made no sense. They noted that genes were expressed within an environment and may have a variable expressions in a different environment, in particular that of prenatal development, and that gene expressions were continuously influenced by the environment through mechanisms such as epigenetics. The epigenetics provides compelling evidence that human behaviors or physiology can be decided by interactions between genes and environments. For example, monozygotic twins usually have exactly same genomes. Scientists focused on comparison studies of twins for evaluating the heritability of genes and roles of epigenetics in divergences and similarity between monozygotic twins, and have proved that epigenetics, which can be influenced by environments, plays an important part in behaviors including stress response.