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Twinning is associated with an increased risk of left-handedness and inverted writing hand posture
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Early Human Development 40 (1994) 23-27
Twinning is associated with an increased risk of left-handedness and inverted writing hand posture Stanley Coren Psychology Department, University of British Columbia, 2136 West Mall, Vancouver, B.C., V6T 124. Canado
Received 10 February 1994; revision received 30 June 1994; accepted 15 August 1994
Abstract Left-handedness and inverted handwriting postures are more frequent among birth-stressed individuals. Because twinning is also associated with increased birth risk, 298 twins were compared to 1192 age and sex matched singletons. Both left-handedness and inversion were
significantly more common in twins, although other indexes of laterality were not affected. Keywork
Handedness; Laterality; Birth stress; Twins
1. Introduction There has been a continuing interest in the etiology of left-handedness. A number of theories of handedness suggest that whether individuals are right- or left-handed is determined, at least in part, by genetic contributions from their parents (see [l] for a review). Unfortunately, data obtained from family studies attempting to establish the heritability of handedness have not strongly supported such theories. While left-handedness seems to be elevated in families where there is a left-handed mother, the patterns of familial resemblance seem stronger for the strength or consistency of handedness, rather than the direction or side preferred [2-41. One review of 13 twin studies on handedness concluded that there was no difference between monozygotic and dizygotic twins in terms of their concordance as to left- or right-handedness. This was interpreted as evidence against a genetic component in handedness [5]. The relative weakness of the data supporting a genetic explanation for manual lateraliza0378-3782/94/$07.00
0
1994 Elsevier Science Ireland Ltd. All rights reserved
SSDI 0378-3782(94)01589-H
24
S. Coren/Early Hum. Dev. 40 (1994) 23-27
tion has led many researchers to seek alternative explanations for the emergence of left-handedness as a phenotype. The most researched non-genetic explanation of left-handedness begins with the observation that sinistrality has been found to be associated with a number of neurological and psychological difficulties [6,7], with immune system problems [6,8], and may even be predictive of reduced life span [5,9,10]. It has been suggested that the mechanism that links left-handedness with various physical and neurological problems is the existence of pre- or perinatal stressors. When risk factors such as low birth weight, prematurity, prolonged labour, birth-related anoxias, Rh incompatibilities, or advanced maternal age at parturition are present, the percentage of left-handed offspring rises [5,1 l-141, and it is argued that the left-handedness may thus be a ‘soft sign’ of some form of neuropathology that resulted from birth stress. Birth stress affects more than just handedness. Other indexes of laterality, such as foot, eye and ear preference may also be susceptible to switching away from the expected dextral dominance, due to birth related pathological factors [14]. Mathematical modelling, however, based upon the distributions of lateral of preference in the population, have suggested that these other indexes may be less sensitive markers of birth trauma [l I]. Apart from handedness, there is another aspect of hand use that appears to be associated with birth stress. Specifically, it has been shown that inverted handwriting posture (where the writer holds the hand in a hooked position with the pen above, rather than below the written line) is more common in individuals with birth complications [ 151. If this reasoning is correct, then an interesting population in which to study handedness would be twins. Twinning is generally viewed as a risk factor since twins are more likely to be born with low birth weights, and twin births are more likely to be associated with having prolonged labor, bleeding, incorrect position at birth or asphyxial problems [16]. To the extent that pregnancy complications are related to increased sinistrality, we should then observe an increased incidence of lefthandedness in twins, and perhaps, if the effects are large enough, we might also find increased left-sidedness in other indexes of laterality. Furthermore, it should also be the case that twins should be more likely to adopt an inverted handwriting position if twinning is serving as a birth stressor. 2. Methods and materials 2. I. Study population The twin population was initially contacted at the Twins Days Festival in Twinsburg, OH. The final sample consisted of 298 individuals, each of whom were members of a twin pair. There were 222 women and 76 men in the sample with a mean age of 36.36 years (SD. 14.63). Of the twin pairs, 74 were dizygotic and 224 were monozygotic. A control sample of 1192 individuals (four singletons for each twin subject) was randomly selected by computer from a normative sample of over 10 000 individuals which forms the data base of the University of British Columbia Neuropsychological Survey. This is a project that has been gathering data on various aspects of human
S. Coren/Early
Hum. Dev. 40 (1994) 23-27
25
laterality since 1976. The only restrictions on subject selection were that the control singletons match the twins in terms of genetic sex and that twins and singletons should be matched in chronological age since both sex and age have been shown to be a factor in the distribution of handedness [4,6,17]. This resulted in a sample of 1192 control subjects, with a mean age of 36.49 years (S.D. 14.59) with 888 women and 304 men. 2.2. Measurement of variables Laterality of hand, foot, eye and ear were measured using the Lateral Preference Inventory [17]. This is a self-report inventory in which each sub-scale involves four items. Extensive norms exist for this scale and it has been shown that there is 96% concordance between the self-reports on this inventory and direct behavioral performance testing for lateral preference in laboratory settings [4,17]. Straight versus inverted handwriting position was determined by having subjects match their handwriting posture against four drawings (inverted left, straight left, inverted right, straight right) a procedure that has been shown to have high concordances with classification by trained observers (98% for straight and 94% for inverted positions)
WI. 3. Results
Each subject was dichotomously classified as left- versus right-sided for hand, foot, eye and ear according to generally accepted procedures. First, a laterality index was computed for each subscale of the inventory, defined as (R - L)IN, where R is the number of ‘right’ responses, L is the number of ‘left’ responses and N is the total number of items used in the scale. In this index, consistent right-sidedness is + 1, consistent left-sidedness is -1 while ambilaterality would be an index of 0. To achieve dichotomous classification, individuals with scores greater than 0 listed as rightsided for that index, while individuals with scores less than or equal to 0 listed as left-sided [4,17]. Since ambilaterality listed with left-sidedness, we are actually scor-
Table I Lateral preference of band, foot, eye and ear in 298 twins compared to 1192 singletons Laterality index
Twins (% left)
Singletons (% left)
Odds ratio
95% confidence interval
Hand Foot Eye Ear
l4.Y 15.2 27.7 45.3
9.9 16.7 26.3 40. I
1.56 0.90 1.08 1.24
I .07-2.26 0.63-I .28 0.81-1.44 0.96-1.60
Twins (% inverted)
Singletons (% inverted)
Odds ratio
95% confidence interval
l6.4*
II.4
1.51
I .05-2.17
Handwriting position
Qifference between twins and singletons significant with P < 0.05.
26
S. Coren /Early Hum. Dev. 40 (1994) 23-27
ing right-sidedness versus non-right-sidedness for hand, foot, eye and ear. In addition, writing hand position was noted and dichotomously scored as inverted or straight. For each of the four indexes of laterality (hand, foot, eye and ear) and for inverted versus straight handwriting position, odds ratios and 95% confidence intervals were computed. These measures appear as Table 1. As can be seen from the table, twins were significantly more likely to be lefthanded than were singletons. The other indexes of laterality (foot, eye and ear preference) do not show significantly increased sinistrality for the multiple birth individuals. Turning to handwriting posture, there is a significantly increased risk of inverted handwriting posture in the twin sample. Both of the significant findings, seem to be independent of the zygosity of the twins. Handedness does not appear to differ for monozygotic versus dizygotic twins (OR, 0.79; 95% CI, 0.39-1.61) nor does handwriting position (OR, 1.25; 95% CI, 0.60-2.62). 4. Discussion The major outcome of this study is the observation that twinning is related to an increased incidence of left-handedness and inverted handwriting posture. The hypothesized mechanism is birth stress due to atypical prenatal conditions which result from multiple fetuses sharing resources and space, and complicating the delivery process [16]. Presumably, normal maturational and neuronal migration patterns may be altered by such factors and may cause a deviation from the expected dextral pattern of hand dominance [6]. None of the genetic theories of handedness seems to have any mechanism to predict a population shift toward sinistrality in twins. If the reason for increased sinistrality in twins is the hypothesized birth stress, then these data also suggest that handedness is a more sensitive marker of minimal birth stress effects than is laterality of foot, eye and ear, since no shift toward sinistrality is observed in these other indexes. This might have been predicted by the mathematical model known as ‘Rare Trait Marker Model’ [ 1l] which has been used to explain why pathological conditions are often found to be associated with relatively rare physical or behavioral traits. That model demonstrates that the rarer the trait, the higher the likelihood that individuals with the rare characteristic (e.g. lefthandedness) will also manifest various forms of pathology. Left-handedness, with a population incidence of around 13%, will thus be a more sensitive index of pathology than left-footedness with an incidence of 19%, left-eyedness at 29% or left-earedness with an incidence of 40% [4,17]. This also may explain why inverted versus straight writing posture, shows a significant relationship with twinning. Previous work had suggested that hand inversion is associated with birth stress [15]. With a population incidence of around 11% for inverted handwriting position [ 191(which is very similar to the incidence of left-handedness in the population) this might have also been predicted by the Rare Trait Marker Model [ 111. In summary, multiple birth seems to be associated with increased incidence of lefthandedness, with twins about half again as likely to be left-handed as singletons. In addition, twins are more likely to have inverted handwriting postures. These results
S. Coren/lkly
Hum. Dev. 40 (1994) 23-27
21
are consistent with the hypothesis that twinning is a risk factor, associated with possible neurological damage which may cause changes in adult handedness and motor coordination patterns. Acknowledgements This study was supported by the Medical Research Council of Canada and the British Columbia Health Care Research Foundation. I would like to thank Karen Donelly, David Wong and the staff of the Human Neuropsychology and Perception Laboratory of the University of British Columbia who assisted in the collection and analysis of these data. References [l] Annett, M. (1985): Left, Right, Hand and Brain: The Right Shift Theory. Lawrence Erlbaum Associates, London. [2] Bryden, M.P. (1982): Laterality: Functional Asymmetry in the Intact Brain. Academic Press, New York. [3] Coren, S. and Porac, C. (1980): Family patterns in four dimensions of lateral preference. Behav. Genet., 10, 333-348. [4] Porac, C. and Coren, S. (1981): Lateral Preferences and Human Behavior. Springer, New York. [5] Coren, S. and Halpem, D.F. (1991): Left-handedness: a marker for decreased survival fitness. Psychol. Bull., 109, 90-106. [6] Coren, S. (1992): The Left-hander Syndrome: The Causes and Consequences of Left-handedness. Free Press, New York and John Murray, London. [7] Coren, S. (1990): Left-handedness: Behavioral Implications and Anomalies. Advances in Psychology, No. 67. North-Holland, Amsterdam. [8] Geschwind, N. and Galaburda, A.M. (1987): Cerebral Lateralization: Biological Mechanisms, Associations, and Pathology. MIT Press, Cambridge, MA. [9] Coren, S. and Halpem, D.F. (1991): Left-handedness: a marker for decreased survival fitness. Psychol. Bull., 109, 90-106. [lo] Halpe.m, D.F. and Coren, S. (1991): Hand preference and life span. N. Engl. J. Med., 324, 998. [l l] Coren, S. and Searleman, A. (1990): Birth stress and left-handedness: The Rare Trait Marker Model. In: Left-handedness: Behavioral Implications and Anomalies. Advances in Psychology, No. 67, pp. 345-353. Editor: S. Coren. North-Holland, Amsterdam. 1121 Coren, S. (1990): Relative risk of left-handedness in offspring as a function of maternal age at parturition. N. Engl. J. Med., 322, 1673. I131 Searleman, A., Porac, C. and Coren, S. (1989): The relationship between birth order, birth stress handedness and lateral preference: a critical review. Psychol. Bull., 105, 397-408. [14] Coren, S., Searleman, A. and Porac, C. (1982): The effects of specific birth stressors on four indexes of lateral preference. Can. J. Psychol., 36, 478-487. [15] Searleman, A., Porac, C. and Coren, S. (1982): The relationship between birth stress and writing hand posture. Brain Cognit., 1, 158-164. [16] Akerman, A. and Fischbein, S. (1991): Twins: are they at risk? A longitudinal study of twins and nontwins from birth to 18 years of age. Acta Genet. Med. Gemellol., 40, 29-40. [17] Coren, S. (1993): The Lateral Preference Inventory for measurement of handedness, footedness, eyedness and earedness: norms for young adults. Bull. Psychonomic Sot., 31, 1-3. [18] Porac, C. and Coren, S. (1981): Lateral preferences and human behavior. Springer, New York. [19] Porac, C., Coren, S. and Searleman, A. (1983): Inverted versus straight handwriting posture: a family study. Behav. Genet., 13, 31 l-320.
Early Human Development 40 (1994) 23-27
Twinning is associated with an increased risk of left-handedness and inverted writing hand posture Stanley Coren Psychology Department, University of British Columbia, 2136 West Mall, Vancouver, B.C., V6T 124. Canado
Received 10 February 1994; revision received 30 June 1994; accepted 15 August 1994
Abstract Left-handedness and inverted handwriting postures are more frequent among birth-stressed individuals. Because twinning is also associated with increased birth risk, 298 twins were compared to 1192 age and sex matched singletons. Both left-handedness and inversion were
significantly more common in twins, although other indexes of laterality were not affected. Keywork
Handedness; Laterality; Birth stress; Twins
1. Introduction There has been a continuing interest in the etiology of left-handedness. A number of theories of handedness suggest that whether individuals are right- or left-handed is determined, at least in part, by genetic contributions from their parents (see [l] for a review). Unfortunately, data obtained from family studies attempting to establish the heritability of handedness have not strongly supported such theories. While left-handedness seems to be elevated in families where there is a left-handed mother, the patterns of familial resemblance seem stronger for the strength or consistency of handedness, rather than the direction or side preferred [2-41. One review of 13 twin studies on handedness concluded that there was no difference between monozygotic and dizygotic twins in terms of their concordance as to left- or right-handedness. This was interpreted as evidence against a genetic component in handedness [5]. The relative weakness of the data supporting a genetic explanation for manual lateraliza0378-3782/94/$07.00
0
1994 Elsevier Science Ireland Ltd. All rights reserved
SSDI 0378-3782(94)01589-H
24
S. Coren/Early Hum. Dev. 40 (1994) 23-27
tion has led many researchers to seek alternative explanations for the emergence of left-handedness as a phenotype. The most researched non-genetic explanation of left-handedness begins with the observation that sinistrality has been found to be associated with a number of neurological and psychological difficulties [6,7], with immune system problems [6,8], and may even be predictive of reduced life span [5,9,10]. It has been suggested that the mechanism that links left-handedness with various physical and neurological problems is the existence of pre- or perinatal stressors. When risk factors such as low birth weight, prematurity, prolonged labour, birth-related anoxias, Rh incompatibilities, or advanced maternal age at parturition are present, the percentage of left-handed offspring rises [5,1 l-141, and it is argued that the left-handedness may thus be a ‘soft sign’ of some form of neuropathology that resulted from birth stress. Birth stress affects more than just handedness. Other indexes of laterality, such as foot, eye and ear preference may also be susceptible to switching away from the expected dextral dominance, due to birth related pathological factors [14]. Mathematical modelling, however, based upon the distributions of lateral of preference in the population, have suggested that these other indexes may be less sensitive markers of birth trauma [l I]. Apart from handedness, there is another aspect of hand use that appears to be associated with birth stress. Specifically, it has been shown that inverted handwriting posture (where the writer holds the hand in a hooked position with the pen above, rather than below the written line) is more common in individuals with birth complications [ 151. If this reasoning is correct, then an interesting population in which to study handedness would be twins. Twinning is generally viewed as a risk factor since twins are more likely to be born with low birth weights, and twin births are more likely to be associated with having prolonged labor, bleeding, incorrect position at birth or asphyxial problems [16]. To the extent that pregnancy complications are related to increased sinistrality, we should then observe an increased incidence of lefthandedness in twins, and perhaps, if the effects are large enough, we might also find increased left-sidedness in other indexes of laterality. Furthermore, it should also be the case that twins should be more likely to adopt an inverted handwriting position if twinning is serving as a birth stressor. 2. Methods and materials 2. I. Study population The twin population was initially contacted at the Twins Days Festival in Twinsburg, OH. The final sample consisted of 298 individuals, each of whom were members of a twin pair. There were 222 women and 76 men in the sample with a mean age of 36.36 years (SD. 14.63). Of the twin pairs, 74 were dizygotic and 224 were monozygotic. A control sample of 1192 individuals (four singletons for each twin subject) was randomly selected by computer from a normative sample of over 10 000 individuals which forms the data base of the University of British Columbia Neuropsychological Survey. This is a project that has been gathering data on various aspects of human
S. Coren/Early
Hum. Dev. 40 (1994) 23-27
25
laterality since 1976. The only restrictions on subject selection were that the control singletons match the twins in terms of genetic sex and that twins and singletons should be matched in chronological age since both sex and age have been shown to be a factor in the distribution of handedness [4,6,17]. This resulted in a sample of 1192 control subjects, with a mean age of 36.49 years (S.D. 14.59) with 888 women and 304 men. 2.2. Measurement of variables Laterality of hand, foot, eye and ear were measured using the Lateral Preference Inventory [17]. This is a self-report inventory in which each sub-scale involves four items. Extensive norms exist for this scale and it has been shown that there is 96% concordance between the self-reports on this inventory and direct behavioral performance testing for lateral preference in laboratory settings [4,17]. Straight versus inverted handwriting position was determined by having subjects match their handwriting posture against four drawings (inverted left, straight left, inverted right, straight right) a procedure that has been shown to have high concordances with classification by trained observers (98% for straight and 94% for inverted positions)
WI. 3. Results
Each subject was dichotomously classified as left- versus right-sided for hand, foot, eye and ear according to generally accepted procedures. First, a laterality index was computed for each subscale of the inventory, defined as (R - L)IN, where R is the number of ‘right’ responses, L is the number of ‘left’ responses and N is the total number of items used in the scale. In this index, consistent right-sidedness is + 1, consistent left-sidedness is -1 while ambilaterality would be an index of 0. To achieve dichotomous classification, individuals with scores greater than 0 listed as rightsided for that index, while individuals with scores less than or equal to 0 listed as left-sided [4,17]. Since ambilaterality listed with left-sidedness, we are actually scor-
Table I Lateral preference of band, foot, eye and ear in 298 twins compared to 1192 singletons Laterality index
Twins (% left)
Singletons (% left)
Odds ratio
95% confidence interval
Hand Foot Eye Ear
l4.Y 15.2 27.7 45.3
9.9 16.7 26.3 40. I
1.56 0.90 1.08 1.24
I .07-2.26 0.63-I .28 0.81-1.44 0.96-1.60
Twins (% inverted)
Singletons (% inverted)
Odds ratio
95% confidence interval
l6.4*
II.4
1.51
I .05-2.17
Handwriting position
Qifference between twins and singletons significant with P < 0.05.
26
S. Coren /Early Hum. Dev. 40 (1994) 23-27
ing right-sidedness versus non-right-sidedness for hand, foot, eye and ear. In addition, writing hand position was noted and dichotomously scored as inverted or straight. For each of the four indexes of laterality (hand, foot, eye and ear) and for inverted versus straight handwriting position, odds ratios and 95% confidence intervals were computed. These measures appear as Table 1. As can be seen from the table, twins were significantly more likely to be lefthanded than were singletons. The other indexes of laterality (foot, eye and ear preference) do not show significantly increased sinistrality for the multiple birth individuals. Turning to handwriting posture, there is a significantly increased risk of inverted handwriting posture in the twin sample. Both of the significant findings, seem to be independent of the zygosity of the twins. Handedness does not appear to differ for monozygotic versus dizygotic twins (OR, 0.79; 95% CI, 0.39-1.61) nor does handwriting position (OR, 1.25; 95% CI, 0.60-2.62). 4. Discussion The major outcome of this study is the observation that twinning is related to an increased incidence of left-handedness and inverted handwriting posture. The hypothesized mechanism is birth stress due to atypical prenatal conditions which result from multiple fetuses sharing resources and space, and complicating the delivery process [16]. Presumably, normal maturational and neuronal migration patterns may be altered by such factors and may cause a deviation from the expected dextral pattern of hand dominance [6]. None of the genetic theories of handedness seems to have any mechanism to predict a population shift toward sinistrality in twins. If the reason for increased sinistrality in twins is the hypothesized birth stress, then these data also suggest that handedness is a more sensitive marker of minimal birth stress effects than is laterality of foot, eye and ear, since no shift toward sinistrality is observed in these other indexes. This might have been predicted by the mathematical model known as ‘Rare Trait Marker Model’ [ 1l] which has been used to explain why pathological conditions are often found to be associated with relatively rare physical or behavioral traits. That model demonstrates that the rarer the trait, the higher the likelihood that individuals with the rare characteristic (e.g. lefthandedness) will also manifest various forms of pathology. Left-handedness, with a population incidence of around 13%, will thus be a more sensitive index of pathology than left-footedness with an incidence of 19%, left-eyedness at 29% or left-earedness with an incidence of 40% [4,17]. This also may explain why inverted versus straight writing posture, shows a significant relationship with twinning. Previous work had suggested that hand inversion is associated with birth stress [15]. With a population incidence of around 11% for inverted handwriting position [ 191(which is very similar to the incidence of left-handedness in the population) this might have also been predicted by the Rare Trait Marker Model [ 111. In summary, multiple birth seems to be associated with increased incidence of lefthandedness, with twins about half again as likely to be left-handed as singletons. In addition, twins are more likely to have inverted handwriting postures. These results
S. Coren/lkly
Hum. Dev. 40 (1994) 23-27
21
are consistent with the hypothesis that twinning is a risk factor, associated with possible neurological damage which may cause changes in adult handedness and motor coordination patterns. Acknowledgements This study was supported by the Medical Research Council of Canada and the British Columbia Health Care Research Foundation. I would like to thank Karen Donelly, David Wong and the staff of the Human Neuropsychology and Perception Laboratory of the University of British Columbia who assisted in the collection and analysis of these data. References [l] Annett, M. (1985): Left, Right, Hand and Brain: The Right Shift Theory. Lawrence Erlbaum Associates, London. [2] Bryden, M.P. (1982): Laterality: Functional Asymmetry in the Intact Brain. Academic Press, New York. [3] Coren, S. and Porac, C. (1980): Family patterns in four dimensions of lateral preference. Behav. Genet., 10, 333-348. [4] Porac, C. and Coren, S. (1981): Lateral Preferences and Human Behavior. Springer, New York. [5] Coren, S. and Halpem, D.F. (1991): Left-handedness: a marker for decreased survival fitness. Psychol. Bull., 109, 90-106. [6] Coren, S. (1992): The Left-hander Syndrome: The Causes and Consequences of Left-handedness. Free Press, New York and John Murray, London. [7] Coren, S. (1990): Left-handedness: Behavioral Implications and Anomalies. Advances in Psychology, No. 67. North-Holland, Amsterdam. [8] Geschwind, N. and Galaburda, A.M. (1987): Cerebral Lateralization: Biological Mechanisms, Associations, and Pathology. MIT Press, Cambridge, MA. [9] Coren, S. and Halpem, D.F. (1991): Left-handedness: a marker for decreased survival fitness. Psychol. Bull., 109, 90-106. [lo] Halpe.m, D.F. and Coren, S. (1991): Hand preference and life span. N. Engl. J. Med., 324, 998. [l l] Coren, S. and Searleman, A. (1990): Birth stress and left-handedness: The Rare Trait Marker Model. In: Left-handedness: Behavioral Implications and Anomalies. Advances in Psychology, No. 67, pp. 345-353. Editor: S. Coren. North-Holland, Amsterdam. 1121 Coren, S. (1990): Relative risk of left-handedness in offspring as a function of maternal age at parturition. N. Engl. J. Med., 322, 1673. I131 Searleman, A., Porac, C. and Coren, S. (1989): The relationship between birth order, birth stress handedness and lateral preference: a critical review. Psychol. Bull., 105, 397-408. [14] Coren, S., Searleman, A. and Porac, C. (1982): The effects of specific birth stressors on four indexes of lateral preference. Can. J. Psychol., 36, 478-487. [15] Searleman, A., Porac, C. and Coren, S. (1982): The relationship between birth stress and writing hand posture. Brain Cognit., 1, 158-164. [16] Akerman, A. and Fischbein, S. (1991): Twins: are they at risk? A longitudinal study of twins and nontwins from birth to 18 years of age. Acta Genet. Med. Gemellol., 40, 29-40. [17] Coren, S. (1993): The Lateral Preference Inventory for measurement of handedness, footedness, eyedness and earedness: norms for young adults. Bull. Psychonomic Sot., 31, 1-3. [18] Porac, C. and Coren, S. (1981): Lateral preferences and human behavior. Springer, New York. [19] Porac, C., Coren, S. and Searleman, A. (1983): Inverted versus straight handwriting posture: a family study. Behav. Genet., 13, 31 l-320.