2003 IFA Congress: Montreal, Canada

Genetic Aspects of Stuttering in South Korea

Moonja Shin1 and Sungeun Lee2
1Shin’s Speech-Language Clinic, Hae S00 B/D IE Socho-Dong, I360-12 Socho-Gu, Seoul, Korea
2Graduate Program in Speech Pathology, Yonsei University, 134 Shinchon-Dong Seodaemurz-Gu , Seoul, Korea


This study investigated the genetic inheritance of 229 stuttering males and females between ages 2 and 45. Subjects were divided into three age groups. 55 percent of subjects reported a family history of stuttering. Females who stuttered were more likely than males who stuttered to have stuttering relatives. Among the preschool and school-age groups, subjects without language difficulties were no more likely than adult subjects to have a family history. Adult subjects without language difficulties were more likely to have a family history. Stuttering-onset ages were earlier for subjects with a family history of stuttering than for subjects without it.

  1. Introduction
The researchers consider that stuttering is influenced by the complex interaction of several factors. These include neuro-physiological, social, psychological, and linguistic factors and it appears that all of these have some influence on the onset and progress of stuttering (Guitar, 1998; Van Riper, 1982). Family studies, twin studies, and adoption studies have been undertaken to clarify the genetic predisposition to stutter (Guitar, 1998; Kidd, 1977, 1984; Poulos & Webster, 1991).

In spite of the limitations and the variety of study methods used to investigate family history, the high occurrence of stuttering in the families of stutterers is constant (Ambrose et al., 1993; Goldberg & Culatta, 1992; Yairi, 1983). This supports genetic factors as a predisposition to stuttering, and indicates directions for continuing and further studies in this area. The variety of methods used for family history studies poses problems when comparing results. These studies have also taken place exclusively in the West, so it is unreasonable to apply the results to Korea where research into the family history of stutterers has not yet been undertaken. However, the data gained from this kind of study can be important when counseling stutterers, and offers a guide for screening high-risk classes of stuttering. It is for this reason that the present study was proposed. The purpose of this study is to investigate the family history of stutterers, whether it is or is not dependent on age group, and to find the relation between language disorder and family history. The present study will also examine the relationship of stuttering with communicative disorder, being limited to speech-language disorders.

  1. Method
    1. Subjects The subjects were selected from stuttering persons who had visited Shin’s Speech-Language Clinic during the past 5 years (Jan. 1996 to Sept. 2001). There were 229 persons (164 males, 35 females) ranging in age from 2 years to 45 years. Criteria for subject selection were as follows: 1) Riley’s Stuttering Severity Instrument severity rating of at least ‘mild’ or more; 2) parental judgment 38 Theory, research and therapy in fluency disorders or self-judgment that the subject stutters. Children who had a less than 6 months onset period of stuttering were also excluded. Potential subjects were asked to report in a case history forrnwhether there are any other individuals in their family background or immediate family who stutter. If they answered yes to this question, they were asked to report which family members stuttered. Verbal questioning provided confirmation in each case. Persons who were uncertain about family history and/or onset age of stuttering were excluded.
    2. Data Collection & Analysis
      The data on subject’s age, gender, stuttering-onset age, other communicative disorders and familial history of stuttering were gathered from the subject or the subject’s parent during an extensive intake interview. For identifying other communicative disorders, language and articulation screening tests were done. Communicative disorders consisted of developmental language delay, expressive language delay, and articulation disorder.
      Subjects were divided into three age groups: the preschool-age group (ages 2 to 6), the school- age group (ages 7 to 18), and adults (aged over 19). The preschool-age group comprised 94 children, the school-age group had 64 students, and there were 71 adults. The three groups were nsirnilar in size. For a more accurate examination of the familial history, subjects’ relatives were divided into subgroups: 1) stuttering relatives’ gender; 2) immediate family or extended family; 3) paternal side or maternal side.
  2. Results
    Fifty-five percent of 229 subjects reported a familial history of stuttering. There were 57 (60.6%) in the preschool-age group, 29 (45.3%) in the school-age group, and 40 (56.3%) in adults.
    1. Gender & Familial history .
      Table 1. shows the relation between gender and familial history according to age group. Females were found to be more likely than males to have stuttering relatives. The absence-to- presence of familial history ratios of having stuttering relatives were 1:1.2 in males and 1:1.5 in females. But no statistically significant difference showed F2(1,229)=0.41, p>0.05). For males, the preschool-age group had the highest percent of familial history. In contrast, the adult group had the highest percentage for females.

      ga t1

      Table 1. Gender distribution for absence and presence of familial history

    2. Communicative disorder & Family history Eighty (34.9%) had communicative disorders as well as fluency disorder. Subjects having communicative disorders were 52 in the preschool-age group, 22 in the school-age group, and 6 in adults. Subjects with communicative disorders were assessed as developmental language delay (20.1%), articulation disorder (5.2%), expressive language delay (4.8%).

      Table 2. summarizes the relation between communicative disorders and familial history according to age group. Among the preschool-age and school-age groups, subjects without language difficulties were no more likely than adult subjects to have a familial history of stuttering. Especially for adults, subjects without a language problem were more likely to have family histories F2(2.64)=7.45, p<0.05). Adults with a language problem were more likely to have no familial history.

      ga t2

      Table 2. Distribution of absence and presence of communicative disorder for absence and presence of familial history

    3.  Stuttering onset ages & Familial history
      Stuttering onset ages were reported ranging in age from 1 year to 16 years. Mean age was 5.3 years. Over 65% of subjects reported stuttering-onset ages from 2 years to 5 years. As shown in Figure 1 accumulation graph, stuttering-onset ages were earlier for subjects with a familial history of stuttering than for subjects without it. The older subjects were at the time of data collection, the earlier onset ages ‘mad been (Figure 2).

      ga f1

      Figure 1. Accumulation of stuttering-onset ages for absence and presence of familial history

    4. Analysis of stuttering relatives
      The highest frequency of relatives represented in order: fathers, mothers, uncles-paternal side, uncles-maternal side, cousins, grandfathers. Eighty-six subjects reported they have stuttering people in their immediate family. Among them they reported father(44) most frequently and mother(27) next most often.

      Figure 2 shows that there were more male relatives (73.8%) than female relatives (26.2%). A higher percentage of male relatives of male subjects (75.5%) was evident than male relatives of 40 Theory, research and therapy in fluency disorders female subjects (65.5%). In other words, there were fewer female relatives of male su ajects (24.5%) than female relatives of female subjects (34.5%).

      ga f2

      Figure 2. Gender distribution of stuttering relatives for subject’s gender

      Stuttering relatives were reported as 41.3% in immediate family, 35.7% in extended family, 23.0% in both. Immediate family included father, mother, brothers, and sisters. Extended family included grandparents(l0), uncles(l7), aunts(2), and cousin males(16) and cousin females(l). Stuttering "relatives were reported as 53.8% in only paternal side, 36.8% in only maternal side, 9.4% in both. In Figure 3, the higher frequency showed both paternal relatives in male subjects and maternal relatives in female subjects.

      ga f3

      Figure 3. Proportion of paternal and maternal relatives for subjectis gender

  3. Discussion
    This study investigated the family trees of subjects visiting Shin’s Speech-Language Clinic over the past five years for the purpose of assessment and treatment. Therefore, there may be some differences from other studies in which the subjects were all stutterers. For example, in the case of adults with stuttering, men are more likely to visit the clinic than women because men are more interested in solving their problem for both career and social reasons. The number of relatives the normal controls have often shows as a distinct difference between normals and stutterers. To obtain more precise data, the subjects of this study were people assessed as having a severity of ‘mild’ or more severe stuttering. In the case of children, the post onset period was more than six months, and children with transient stuttering were excluded.

    The results of this study are very similar to other foreign data on heredity. The probability that stutterers have family members or relatives with stuttering is 42-46% in the case of foreign studies (Ambrose, Yairi & Cox, 1993; Ambrose et al., 1983; Goldberg & Culatta, 1992), but it is slightly lower in this study. This might be influenced by the fact that Korean subjects did not want to reveal the presence of family members with stuttering. In fact, there was one subject who only admitted that he had a family member with stuttering after he had been receiving treatment for one year. The data including cousins show that 55% of stutterers have a family history of fluency disorder, and this is also a lower incidence compared to studies done in other countries (64 -71%)(Ambrose et al., 1993; Goldberg & Culatta, 1992; Poulos & Webster, 1991; Wepman, 1939; Yairi, 1983). The subjects’ family trees were restricted to cousins in this study to exclude indistinct data, whereas the range of subjects is generally wider in overseas studies and frequently includes extended family as distinct from immediate family. However, because of the possibility of disposition and cultural differences between Korean and western societies, more studies are necessary. This study may demonstrate the physiological heredity factor of fluency disorder better than overseas studies however, because Korea has a low ratio of adoption.

    The probability that a stutterer has more than two stuttering family members is 34.9% (44 subjects), and this means that the stuttering heredity factor can exist in several members of one generation. There are cases where great grandparents stuttered but parents did not, which suggested the possibility that the inheritability factor for fluency disorder can skip a generation. A group with a family tree of stuttering has a lower age of post onset than the opposite group. This suggested two possibilities; parents whose family already includes stutterers may respond earlier, or this group starts stuttering earlier. Women have a higher probability of having stuttering relatives than men, and this result corresponds to the findings of previous studies. However, it is not meaningful statistically. Women have a higher probability of a mother or a maternal family line of stuttering, so stuttering appears to be inherited differently according to gender.

    In the stuttering adult group, those without language problems had a stronger family history of stuttering. This result is related to the study of Poulos and Webster (1991) who suggested that there could be subgroups of stuttering; that is, groups with hereditary stuttering and language disorders related to stuttering or neurological impairment. However, more data and discussion are required to validate this view.

    In spite of the limitations mentioned, the results of this study suggest that a predisposition to stuttering can be transmitted genetically. However, as previous studies have made clear it is impossible to know precisely which gene is responsible for the transmission, and the physical differences between stutterers and non-stutterers remain unclear. This study data suggest that genetic factors exist in stuttering, but they do not identify some relationship between genetic aspects and recovery of stuttering.
    In this study, it appears that the more age increases, the more females recover, with the difference in the male-to-female ratio increasing. But the female adult subjects had a_higher genetic probability than did the other subjects. It is possible to presuppose that genetic aspects influence somewhat negatively the maintenance or recovery of stuttering in the case of the females.

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