No single, exclusive cause of developmental stuttering is known. A variety of hypotheses and theories suggests multiple factors contributing to stuttering.[1] Among these is the strong evidence that stuttering has a genetic basis.[20] Children who have first-degree relatives who stutter are three times as likely to develop a stutter.[21] However, twin and adoption studies suggest that genetic factors interact with environmental factors for stuttering to occur,[22] and many people who stutter have no family history of the disorder.[23] There is evidence that stuttering is more common in children who also have concurrent speech, language, learning or motor difficulties.[24] Robert West, a pioneer of genetic studies in stuttering, has suggested that the presence of stuttering is connected to the fact that articulated speech is the last major acquisition in human evolution.[25]