A sibling is one of two or more individuals having one or both parents in common. A female sibling is a sister and a male sibling is a brother. In most societies throughout the world, siblings often grow up together, thereby facilitating the development of strong emotional bonds. The emotional bond between siblings is often complicated and is influenced by factors such as parental treatment, birth order, personality, and personal experiences outside the family. However, there are cases where siblings grow up in separate homes, in different environments. It is known that both nature and nurture figure in development; researchers are attempting to ascertain just which one plays the larger role. Identical twins share 100% of their DNA. Full siblings are first-degree relatives and, on average, share 50% of their genes out of those that vary among humans. Half-siblings are second-degree relatives and have, on average, a 25% overlap in their human genetic variation.