راهکارهای درمانی خاص در درمان لکنت زبان کلینیک تخصصی لکنت کرج دولت آباد- بلوار فیروزبخت
nongenetic factors, too must be involved in some cases at least. Felsenfeld (1997) suggests that by careful examination of the histories of each twin we can learn which nonshared variables are signifi¬cant factors in the different fluency outcomes. She cites as a hypothetical example the possibility that one twin might have experienced a serious medi¬cal trauma as an infant. Although twin studies cannot definitively show what these factors are, a more recent study on nearly 4000 unselected twin pairs (Andrews, Morris-Yates, Howie, & Martin, 1990) in a nonclinical study esti¬mated that 71 percent of the variance can be explained by genetic factors, and 21 percent by environmental influences. Very similar heritability (70 percent) was found in a more recent twin study which used a similar statistical analysis procedure (Felsenfeld, Kirk, Zhu, Statham, Neale, & Martin, 2000). Clearly, more research is needed, but the body of evidence from twin studies supports the notion that genetic factors play a significant part in the cause of stuttering.
One very effective way of attempting to limit the potential of environmental influences on genetic studies is to look at the development of children who, having been separated from their natural parents at an early age, have still gone on to develop a stutter. Lack of contact between offspring and parent will mean that any shared stuttering between the two can be attributed to genetic influence. The problem is that adoption studies are few and far between, due largely to the difficulty in accessing adoption records and (like longitudinal studies in treatment efficacy) the time and resources needed to implement a design which is sufficiently well controlled. Felsenfeld (1995) reports some preliminary data from a small cohort of subjects which showed that having a biological parent with a speech language disorder placed the offspring at greater risk for developmental speech disorders (in which stutter¬ing was included) than being raised by a nonbiological parent who had a positive history. However, the fact that small sample sizes were used together with other methodological issues means these data should be interpreted with caution.