Sibling reentered English in 1903 as a scientific coinage
Siblings share at least one parent; brothers are male, sisters female, and a person with none is an only child. The word itself fell out of use after 1425 until an article in Biometrika revived it in 1903 to translate the German Geschwister.
Full siblings share both biological parents and are first-degree relatives, about 50% related, while half-siblings share one parent and are second-degree relatives at about 25%. Maternal half-siblings are called uterine and paternal ones agnate, or consanguine in legal usage. Old English common law once gave half-siblings only half the share of full-blood siblings from an intestate estate, a rule abolished in England but still found in Florida.
Rarer categories abound. Three-quarter siblings share one parent while their other parents are closely related; aviator Charles Lindbergh had children by Brigitte and Marietta Hesshaimer, who were sisters from Germany. Diblings share a sperm or egg donor, milk siblings were nursed by the same woman, a kinship recognized in Islamic law, and cross-siblings are linked only through half-siblings on different sides.
Actual DNA sharing varies. Because chromosomes cross over only a few times, full siblings share on average 50.28% of their DNA with a standard deviation of 3.68%, so a quarter of pairs share more than 52.76%. Fraternal twins are no more alike than ordinary siblings, whereas identical twins come from one zygote and are genetically identical, which makes twin studies useful for separating genes from environment.
Popular belief holds that firstborns are conservative achievers, middle children mediators, and youngest children charmers, but studies have not produced consistent findings, and many psychologists dismiss such claims as pseudo-psychology. Francis Galton’s English Men of Science (1874) observed that eminent scientists and composers were disproportionately firstborn, crediting primogeniture, extra responsibility, and more attention; Alfred Adler later tied birth order to personality.
Source: Sibling
