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Individual and familial patterns in four dimensions of lateral preferences
hbroprychologia. Vol. 17. pp. 543 IO 546. D Pcrgnmon Press Ltd. 1979. Printed in Great Britain.
NOTE INDIVIDUAL
AND
FAIMILIAL
PATTERNS
OF LATERAL CLAM
Department
IN FOUR
DIIMENSIONS
PREFERENCE PORAC
of Psychology, University of Victoria, Victoria, British Columbia, V8W 2Y2, Canada and CORES
STANLEY
Department
of Psychology, University of British Columbia, Vancouver, British Columbia, V6T 1W.5. Canada (Received 20 February 1979)
Abstract-Laterality of hand, foot, eye and ear were assessed for biologically-related individuals. Although mother-offspring resemblances were found for hand and ear, the pattern of correlation does not conform to that expected on the basis of genetic determination. All four indices of laterality seem to be correlated; however, the difference in the degree of association among the forms of laterality suggests that a single causal mechanism is unlikely to account for all manifestations of lateral preference of limb and sense organ.
INDIVIDUAL
AND
FAMILIAL
PATTERNS
OF LATERAL
IN FOUR
DIMENSIONS
PREFERENCE
ALTHOUGH the human body is bilaterally symmetrical in appearance, there are many behavioral coordinations which show lateral asymmetries. The most commonly acknowledged form of lateral dominance is handedness, but lateral preferences are also found for the feet and for the paired sensory organs [I, 21. Existing data indicate that for hand, foot, eye and ear use the population is strongly biased toward the right [3]. Contemporary debates on lateral asymmetries in human coordinations have focused upon three related issues. First, what is the mechanism or mechanisms which support lateral preferences? Second, why is the population predominantly right-sided and thirdly, why do some individuals deviate from the norm of dextrality? Much of the current theorizing suggests that laterality is based upon underlying physiological asymmetries. One frequent argument is that the dominance of the left hemispherein most individuals accounts for the predominance of right-sidedness within the population [4]. An alternative explanation suggests that there is a maturational gradient from left to right in the developing embryo which allows the left side of the central nervous system to develop first, giving it a predominance over the right side. Because of the contralateral nature of neural control, behavioral lateral dominance is thus biased toward the right side of the body [S, 61. Alternate hypotheses suggest genetic determinants for laterality. For example, Annett [7-91 suggests that when the gene for left cerebral dominance is not present, random factors influence whether the individual will be lateralized right or left. LEVY and NACYLAKI [IO] maintain that there are separate alleles for cerebral dominance and neural control with left cerebral dominance and contralateral control being the dominant traits. In contrast to these genetic theories are those that suggest that specific training and experience are major factors in the development of patterns of laterality [I 1-l 31. There has been experimental evidence both for and against each of the above hypotheses. The inconclusive results from the empirical tests may have arisen from the fact that, although theorists attempt to address themselves to all facets of human lateral preference, most of the data deal only with hand preference. If one only measures handedness, there is no way to assess the consistency of the various manifestations of lateral preference within individuals. The mechanisms of causation proposed by the existing hypotheses postulate a high degree of consistency among the four dimensions of laterality. Thus the investigation of both limb and sensory organ preference should not only allow one to assess the adequacy of any one theory but also provide information on whether or not laterality is best thought of as a unitary phenomenon.
- 543
NOTE
5-M
The present study measures laterality of hand, foot, eye and ear in a large sample of biologically-related individuals in an attempt to assess the rntcrrelationships among the diRerent forms of laterality. Also, such family data permit one to test for genetic components which could Increase the resemblance between related individuals on these measures.
METHOD A sample of 701 individuals were drawn from 207 families. Only observers vvho had full use of both hands, feet, eyes and ears, were utilized in the sample. In order to ensure relative homogeneity of socio-economic factors. the sample of families which was studied was selected from among the faculty and professional staff of the University of British Columbia. Biological relatedness was established by means of self-report items.
In order to assess lateral preference for hand, foot, eye and ear, a self-report inventory was prepared based upon a series of items which have been behaviorally validated in two previous studies [14, 151. Behavioral validation was obtained by assessing the degree of concordance between the classification of laterality as
Table
I. Self-report
measures
of lateral
preference
used in the present
study Per cent concordance with performance measures
Hand I. 2. 3. 4.
preference With which With which With which With which
hand do you use an eraser on paper’? hand do you remove the top card when dealing? hand do you throw a ball to hit a target? hand do you draw’?
Eye preference (sighting preference) I. Which eye would you use to peep through a keyhole? 2. If you had to look into a dark bottle to see how full it was, which you use? 3. Which eye would you use to sight down a rifle? Foot preference I. With which foot do you kick a ball? 2. If you had to step onto a chair, which
foot would
100 100 100 100
82.6 eye would
you place in the chair
82.0 76.5
first?
Ear preference 1. If you wanted to listen in on a conversation going on behind a closed door, which ear would you place next to the door to better hear what was being said? 2. Into which ear would you place the earphone of a transistor radio?
97.7 89.8
94.4
87.2
indicated by subjects’ self-report responses with that which was indicated by their scores on performance tests of laterality. All items used in the present inventory had high degrees of classification concordance. The self-report items used in the present study are reported in Table I along with their respective classification concordance rates.
RESULTS
AND
DISCUSSION
A composite of every individual’s scores on each of the four indices of laterality was formed. Each response of “right” was assigned a score of + I, each response of “left” was assigned a score of - I, and each ambilateral response was assigned a 0. The score for each manifestation of laterality was the algebraic sum of these values. This method of scoring incorporates not only the direction but also the strength of the lateral preference.
NOTE
545
Famil,v patterns of fateraliry Correlation coefficients were computed between parents and offspring and between siblings. In families where there was more than one offspring of the designated sex, one child was randomly selected for analysis when the correlation coefficients were computed. This procedure ensured that each family was represented only once in any correlation and, thus, controlled for any bias which is introduced by differences in family size. Similarly, when computing the correlations between sibling pairs in situations where families contained three or more children, only two of these were randomly selected for analysis. Pearson product-moment correlation coefficients were obtained in this manner for all four dimensions of lateral preference. All of the values reported here are partial correlations which control for the effects of the age of both family members. This eliminates the contribution of any possible age biases in lateral preference behaviors [16, 171. The results of these analyses are shown in Table 2. Table
2. Familial
correlations
for four dimensions
Hand Mother-daughter Mother-son Father-daughter Father-son Sib-sib
(No. of pairs (No. of pairs
(No. of pairs
(No. of pairs
(No. of pairs
= 123)
= 95) = 126)
= 104)
= 166)
of lateral
Lateral Eye
preference
preference
type Foot
Ear
0.24*
0.02
0.06
0.07
0.2ot
0.05
0.01
0.167
-0.01 0.12 -0.02
0.05
-0.06
0.15
-0.1
-0.06
-0.03 I
0.05
0.14 0.10
*P < 0.01. tP 5 0.05. Table 2 indicates that there are very few significant familial correlations. The only values that reach statistical significance are the mother-offspring correlations in hand preference and the mother-son correlations in ear preference. Familial resemblances may arise as a product of either the shared environment within a family or the shared biological and genetic structures, or a combination of both forces [IS, 191. Therefore, in assessing possible genetic determinism, it is important to look at the full pattern of familial resemblances and not just those which may exist between parents and offspring. The genetic commonality between biologically-related parents and their offspring and between full siblings is assumed to be 50%. Therefore, in looking at a familial correlation matrix, one would expect to find both parent-offspring and sibling correlations approaching the 0.5 value if there is complete genetic determination of a trait. Although such factors as dominance within a genotype can reduce this expected value, none of the correlations obtained in this study come close to approaching the expected correlation. The present data provide no evidence for significant familial resemblances in eye and foot preference. Although some significant family patterns have emerged, they are also not strong enough to demonstrate clearly a genetic influence. For example, there are significant mother-offspring relationships in handedness, a tinding that is consistent with previous reports [S, 9,201. However, significant mother-child similarities in the absence of father-child and sibling resemblances do not necessarily indicate the genetic transmission of a trait [IfI]. Thus, these significant mother-offspring correlations in handedness could reflect asymmetries in some aspect of the mother-child relationship. For example, some suggestion has been made that mothers attempt to train handedness patterns in their children [20]. There is an alternative way of looking at these data. The use of the correlation coefficient assumes that the laterality of an individual can be characterized by its strength as well as by its direction. In other words, two individuals may be classified as dextral, although one might be slightly more sinistral than the other. However, one could argue that lateral preference is a dichotomous variable. If this is assumed, then the appropriate analysis would be to classify parents as being right or left dominant and to look at the pattern of laterality resulting from particular parental pairings. Given a genetic contribution to forms of lateral preference, one would expect to find an increase in the incidence of right-handedness, for example, among offspring of two right-handed parents over that seen in offspring of parental pairs that include at least one left-hander. The same reasoning could be applied to the other three dimensions of lateral preference as well. To test this notion, each parent and offspring were classified as either right- or left-sided on each of the four indices of preference (individuals whose scores did not show a sidedness bias were included as part of the sinistral group). Each parental pair was then divided into two groups. Those in which both partners were right-sided formed one group, whilst pairings in which one or both partners were left-sided formed the other group. As in the previous analysis each family appeared only once in any set of measures. Where there was
NOTE
546
Table 3. Percentage of right-sided offspring as a function of parental laterality Per cent right-sided offspring Lateral preference One or both Both parents right sided parents left-sided type
X2
Hand Eye Foot
93.8 73.7 93.0
73.1 69.6 87.0
6.95t 0.38 0.11
Ear
81.8
59.6
5.59’
*P < 0.01. tP < 0.05. more than one offspring in a family, one of these was randomly selected for inclusion in the analyses. The results of this procedure are shown in Table 3. As can be seen from this table, the effect of mating pairs is significant for both hand and ear preference while there is no effect for eye or foot preference. Thus the total pattern of this analysis is consistent with the correlational findings. The present data suggest that at least two of the indices of lateral preference (hand and ear) have causal components that can be traced to the common factors shared by family members. However, if there were a common underlying causative process, one would expect that the patterns of familial resemblance would be similar for all four indices of lateral preference. This does not appear to be the case since handedness and earedness show relationships which eye and foot preference do not. Individual parrems of laferulity
If there is a common factor underlying all indices of lateral preference, one would expect certain conditions to hold. For instance, if an individual were right-handed, the probability that he would also be right-footed, right-eyed and right-eared would be higher than if he were left-handed. This expectation suggests that there should be significant correlations between the four indices of laterality when measured within individuals. To assess the degree of relationship among the various forms of laterality, data from the entire group of 701 individuals were used. The laterality scores were computed as they were for the correlational analysis of familial resemblance. Then intercorrelations were generated among the four measures. These results are shown in Table 4. All of the paired correlations are significant with P < 0.001. Table 4. Intercorrelations
Eye
between measures of laterality* (IV = 701)
Eye
Hand
Foot
Ear
I .oo
0.29
0.25 0.48
0.19 0.23 0.29
Hand Foot Ear *All correlations with P < 0.001.
I .oo
I .oo
1.00 in this matrix are significant
The existence of significant correlations among the four indices of laterality seems to suggest that there is some common factor that underlies lateral preference. However, the sizes of the correlations indicate that the contribution of such a factor may be quite small. The largest correlation is that between handedness and footedness. This correlation is quite sizeable and accounts for 23 % of the common variance. All of the other correlations, although significant, are quite small. Each of these only accounts for between 4 and 8 % of the predicted variance. This would seem to suggest that there is a strong common mechanism determining laterality of hand and foot, while the other manifestations of laterality have only a small degree of commonality. To the extent that there is a single common mechanism underlying all forms of lateral preference, all of the
NOTE
547
obtained correlations should not only have been significant and positive but also of approximately the same magnitude. However, this is not the case for these data. The correlation between handedness and footedness is significantly greater than that found for hand and ear and hand and eye. It is also significantly greater than the correlation between foot and eye and foot and ear (all of these obtained differences are significant at P < 0.001). In addition, the correlation between eyedness and earedness seems to be weaker than the other obtained relationships [‘I]. This pattern of results seems to imply that there is a strong common factor supporting the preference of hand and foot that is not shared by the other dimensions of laterality. One can explain the strength of the relationship between handedness and footedness. The neural control of a limb seems to rest primarily with the cerebral hemisphere that is contralateral to it. Hence, the correlation between handedness and footedness may reflect the involvement of a dominant cerebral hemisphere. It is more difficult to implicate this mechanism in the formation of the laterality of the eye and the ear since each eye and ear is bilaterally represented in the cortex. These data suggest several answers to questions concerning the mechanisms which support lateral preference in humans. There is some suggestion that the lateral preferences of the hand and of the ear have a familial component, although whether or not this component is genetic remains unresolved. Furthermore, the asymmetrical nature of the relationships amongst the various indices of laterality, in conjunction with the different patterns of familial resemblances among the measures, suggests that there is probably more than one mechanism that is responsible for the development and the maintenance of lateral preference behaviors. AcA-truwler/~ettlenrs-We acknowledge the assistance of R. CROPP and J. PORAC and the financial assistance of the Natural Sciences and Engineering Research Council of Canada and the Medical Research Council of Canada. This work represents an equal and shared contribution of both authors.
REFERENCES eye. Psychol. Bull. 83. 880-897, 1976. I. PORAC, C. and COREN, S. The dominant Latetnli/.s characteristics and their edrtcatiotral ittrplications. University 2. CLARK, M. M. Left-handedness: of London Press, London, 1957. between lateral preference behaviors in humans. Behav. Bruin 3. PORAC, C. and COREN, S. Relationships Sri. 2, 31 l-312, 1978. pp. 4. LEVY, J. In Hettrisphere /rrtrctiotl in fhe hutt~atr brain, S. J. DI~IOND and J. G. BEAU~~ONT (Editors), 121-183. Elek Science, London, 1974. basis of human laterality. Behav. Bruin Sri. 2, 5. COREALLIS, M. C. and MORGAN, M. J. On the biological 261, 1978. in the nervous system, S. HARSARD, R. W. DOTY, J. JAYIVES, L. GOLD6. MORGAN, M. J. In Lafetuli:ation STEINand G. KRAUTHAMER (Editors), pp. 173-194. Academic Press, New York, 1977. of handedness and cerebral dominance. Nafllre 204, 59-60, 1964. 7. ANNETT, M. A model of the inheritance in families. Ann. Huttr. Gener., Lond. 37, 93-105, 1973. 8. ANSETT, M. Handedness influences on handedness. Behav. Gene{. 8, 227-249, 1978. 9. ASSETT, M. Genetic and non-genetic Gener. 72, 117-128. 1972. 10. LEVY, J. and NAGYLAKI, T. A model for the genetics of handedness. Association, New York, 1946. I I. Blau, A. Tire Master Hand. American Orthopsychiatric mice live in right-handed worlds. Science 187, 181-184, 1975. 12. COLLISS, R. L. When left-handed in the trervous system, S HARNARD, R W. DOTY, J. JAYNES, L. GOLD13. COLLINS,R. L. In Lateralization STEINand G. KRAUTHAMER (Editors), pp. 137-150. Academic Press, New York, 1977. items for the measurement of lateral 14. COREN, S. and PORAC, C. The validity and reliability of self-report preference. Br. J. Psychol. 55, 208-213, 1978. questionaire 15. RACZKOWSKI, D., KALAT, J. W. and NEBES, R. Reliability and validity of some handedness items. Newopsc~~hologia 12, 43-48, 1974. of lateral preference. Paper presented at the 16. PORAC. C. and CORES, S. Age trends in manifestations meetings of the Canadian Psychological Association, Toronto, Ontario, June 1976. behaviors a manifestation of a single underlying process? 17. CORES, S. and PORAC. C. Are laterality Paper presented at the meetings of the Canadian Psychological Association, Ottawa, Ontario, June 1978. 18. FALCOSER, D. S. Introduction to Qltantita/ive Genetics. Ronald Press, New York, 1972. W. H. Freeman, San Francisco, 19. MCCLEARN, G. E. and DE FRIES,J. C. Inrroducrion IO BehavioralGenetics. 1973. a family study. Am. J. Hum. Gene,. 11, 52-62, 1959. 20. FALEK, A. Handedness: J. L. and KINTZ, B. L. Cotnputational Hundbook of Sfatisfics. Scott Foresman. Glenview, 21. Bruning, Illinois, 1968.
NOTE
0" a etabli la lateralit du pied, de la main, de l'oeil et de l'oreille chez des individus biologiquement relies. bien que des ressemblances v&s
entre les meres et leurs descendants
ont BtB trou-
pour la main et l'oreille,
le type des corrOlations ne se conforme pas a celui qu'on attendrait s'il s'agissait de determination
gln@tique.
Les 4 indices de lateralit
dant, la diffgrence
semble Etre correles, cepen-
dans le degr6 d'association
entre les formes de
la lat6ralit6 suggere qu'il est improbable qu'un mdcanisme causal unique rende compte de l'ensemble des manifestations t&ale, membres et organes des sens.
de la prefgrence
la-
Deutschsprachige Zusammenfassung: Seitendominanz von Hand, FUS, Auge und Ohr wurde bestimmt bei biologisch verwandten Individuen. Obwohl Ahnlichkeiten mit der Mutter beziiglichHand- und Ohrdominanz gefunden wurden, entspricht das Korrelationsmuster nicht dem aufgrund genetischer Determinierung erwarteten. Alle vier Seitigkeitsindices scheinen zu interkorrelieren; das AusmaB der Verbindung der einzelnen Lateralitatsformen deutet allerdings darauf hin, daR ein einziger kausaler Mechanismus bei der Manifestation von Clied- und Sinnesorgan-Seitenpraferenzen unwahrscheinlich ist.
NOTE INDIVIDUAL
AND
FAIMILIAL
PATTERNS
OF LATERAL CLAM
Department
IN FOUR
DIIMENSIONS
PREFERENCE PORAC
of Psychology, University of Victoria, Victoria, British Columbia, V8W 2Y2, Canada and CORES
STANLEY
Department
of Psychology, University of British Columbia, Vancouver, British Columbia, V6T 1W.5. Canada (Received 20 February 1979)
Abstract-Laterality of hand, foot, eye and ear were assessed for biologically-related individuals. Although mother-offspring resemblances were found for hand and ear, the pattern of correlation does not conform to that expected on the basis of genetic determination. All four indices of laterality seem to be correlated; however, the difference in the degree of association among the forms of laterality suggests that a single causal mechanism is unlikely to account for all manifestations of lateral preference of limb and sense organ.
INDIVIDUAL
AND
FAMILIAL
PATTERNS
OF LATERAL
IN FOUR
DIMENSIONS
PREFERENCE
ALTHOUGH the human body is bilaterally symmetrical in appearance, there are many behavioral coordinations which show lateral asymmetries. The most commonly acknowledged form of lateral dominance is handedness, but lateral preferences are also found for the feet and for the paired sensory organs [I, 21. Existing data indicate that for hand, foot, eye and ear use the population is strongly biased toward the right [3]. Contemporary debates on lateral asymmetries in human coordinations have focused upon three related issues. First, what is the mechanism or mechanisms which support lateral preferences? Second, why is the population predominantly right-sided and thirdly, why do some individuals deviate from the norm of dextrality? Much of the current theorizing suggests that laterality is based upon underlying physiological asymmetries. One frequent argument is that the dominance of the left hemispherein most individuals accounts for the predominance of right-sidedness within the population [4]. An alternative explanation suggests that there is a maturational gradient from left to right in the developing embryo which allows the left side of the central nervous system to develop first, giving it a predominance over the right side. Because of the contralateral nature of neural control, behavioral lateral dominance is thus biased toward the right side of the body [S, 61. Alternate hypotheses suggest genetic determinants for laterality. For example, Annett [7-91 suggests that when the gene for left cerebral dominance is not present, random factors influence whether the individual will be lateralized right or left. LEVY and NACYLAKI [IO] maintain that there are separate alleles for cerebral dominance and neural control with left cerebral dominance and contralateral control being the dominant traits. In contrast to these genetic theories are those that suggest that specific training and experience are major factors in the development of patterns of laterality [I 1-l 31. There has been experimental evidence both for and against each of the above hypotheses. The inconclusive results from the empirical tests may have arisen from the fact that, although theorists attempt to address themselves to all facets of human lateral preference, most of the data deal only with hand preference. If one only measures handedness, there is no way to assess the consistency of the various manifestations of lateral preference within individuals. The mechanisms of causation proposed by the existing hypotheses postulate a high degree of consistency among the four dimensions of laterality. Thus the investigation of both limb and sensory organ preference should not only allow one to assess the adequacy of any one theory but also provide information on whether or not laterality is best thought of as a unitary phenomenon.
- 543
NOTE
5-M
The present study measures laterality of hand, foot, eye and ear in a large sample of biologically-related individuals in an attempt to assess the rntcrrelationships among the diRerent forms of laterality. Also, such family data permit one to test for genetic components which could Increase the resemblance between related individuals on these measures.
METHOD A sample of 701 individuals were drawn from 207 families. Only observers vvho had full use of both hands, feet, eyes and ears, were utilized in the sample. In order to ensure relative homogeneity of socio-economic factors. the sample of families which was studied was selected from among the faculty and professional staff of the University of British Columbia. Biological relatedness was established by means of self-report items.
In order to assess lateral preference for hand, foot, eye and ear, a self-report inventory was prepared based upon a series of items which have been behaviorally validated in two previous studies [14, 151. Behavioral validation was obtained by assessing the degree of concordance between the classification of laterality as
Table
I. Self-report
measures
of lateral
preference
used in the present
study Per cent concordance with performance measures
Hand I. 2. 3. 4.
preference With which With which With which With which
hand do you use an eraser on paper’? hand do you remove the top card when dealing? hand do you throw a ball to hit a target? hand do you draw’?
Eye preference (sighting preference) I. Which eye would you use to peep through a keyhole? 2. If you had to look into a dark bottle to see how full it was, which you use? 3. Which eye would you use to sight down a rifle? Foot preference I. With which foot do you kick a ball? 2. If you had to step onto a chair, which
foot would
100 100 100 100
82.6 eye would
you place in the chair
82.0 76.5
first?
Ear preference 1. If you wanted to listen in on a conversation going on behind a closed door, which ear would you place next to the door to better hear what was being said? 2. Into which ear would you place the earphone of a transistor radio?
97.7 89.8
94.4
87.2
indicated by subjects’ self-report responses with that which was indicated by their scores on performance tests of laterality. All items used in the present inventory had high degrees of classification concordance. The self-report items used in the present study are reported in Table I along with their respective classification concordance rates.
RESULTS
AND
DISCUSSION
A composite of every individual’s scores on each of the four indices of laterality was formed. Each response of “right” was assigned a score of + I, each response of “left” was assigned a score of - I, and each ambilateral response was assigned a 0. The score for each manifestation of laterality was the algebraic sum of these values. This method of scoring incorporates not only the direction but also the strength of the lateral preference.
NOTE
545
Famil,v patterns of fateraliry Correlation coefficients were computed between parents and offspring and between siblings. In families where there was more than one offspring of the designated sex, one child was randomly selected for analysis when the correlation coefficients were computed. This procedure ensured that each family was represented only once in any correlation and, thus, controlled for any bias which is introduced by differences in family size. Similarly, when computing the correlations between sibling pairs in situations where families contained three or more children, only two of these were randomly selected for analysis. Pearson product-moment correlation coefficients were obtained in this manner for all four dimensions of lateral preference. All of the values reported here are partial correlations which control for the effects of the age of both family members. This eliminates the contribution of any possible age biases in lateral preference behaviors [16, 171. The results of these analyses are shown in Table 2. Table
2. Familial
correlations
for four dimensions
Hand Mother-daughter Mother-son Father-daughter Father-son Sib-sib
(No. of pairs (No. of pairs
(No. of pairs
(No. of pairs
(No. of pairs
= 123)
= 95) = 126)
= 104)
= 166)
of lateral
Lateral Eye
preference
preference
type Foot
Ear
0.24*
0.02
0.06
0.07
0.2ot
0.05
0.01
0.167
-0.01 0.12 -0.02
0.05
-0.06
0.15
-0.1
-0.06
-0.03 I
0.05
0.14 0.10
*P < 0.01. tP 5 0.05. Table 2 indicates that there are very few significant familial correlations. The only values that reach statistical significance are the mother-offspring correlations in hand preference and the mother-son correlations in ear preference. Familial resemblances may arise as a product of either the shared environment within a family or the shared biological and genetic structures, or a combination of both forces [IS, 191. Therefore, in assessing possible genetic determinism, it is important to look at the full pattern of familial resemblances and not just those which may exist between parents and offspring. The genetic commonality between biologically-related parents and their offspring and between full siblings is assumed to be 50%. Therefore, in looking at a familial correlation matrix, one would expect to find both parent-offspring and sibling correlations approaching the 0.5 value if there is complete genetic determination of a trait. Although such factors as dominance within a genotype can reduce this expected value, none of the correlations obtained in this study come close to approaching the expected correlation. The present data provide no evidence for significant familial resemblances in eye and foot preference. Although some significant family patterns have emerged, they are also not strong enough to demonstrate clearly a genetic influence. For example, there are significant mother-offspring relationships in handedness, a tinding that is consistent with previous reports [S, 9,201. However, significant mother-child similarities in the absence of father-child and sibling resemblances do not necessarily indicate the genetic transmission of a trait [IfI]. Thus, these significant mother-offspring correlations in handedness could reflect asymmetries in some aspect of the mother-child relationship. For example, some suggestion has been made that mothers attempt to train handedness patterns in their children [20]. There is an alternative way of looking at these data. The use of the correlation coefficient assumes that the laterality of an individual can be characterized by its strength as well as by its direction. In other words, two individuals may be classified as dextral, although one might be slightly more sinistral than the other. However, one could argue that lateral preference is a dichotomous variable. If this is assumed, then the appropriate analysis would be to classify parents as being right or left dominant and to look at the pattern of laterality resulting from particular parental pairings. Given a genetic contribution to forms of lateral preference, one would expect to find an increase in the incidence of right-handedness, for example, among offspring of two right-handed parents over that seen in offspring of parental pairs that include at least one left-hander. The same reasoning could be applied to the other three dimensions of lateral preference as well. To test this notion, each parent and offspring were classified as either right- or left-sided on each of the four indices of preference (individuals whose scores did not show a sidedness bias were included as part of the sinistral group). Each parental pair was then divided into two groups. Those in which both partners were right-sided formed one group, whilst pairings in which one or both partners were left-sided formed the other group. As in the previous analysis each family appeared only once in any set of measures. Where there was
NOTE
546
Table 3. Percentage of right-sided offspring as a function of parental laterality Per cent right-sided offspring Lateral preference One or both Both parents right sided parents left-sided type
X2
Hand Eye Foot
93.8 73.7 93.0
73.1 69.6 87.0
6.95t 0.38 0.11
Ear
81.8
59.6
5.59’
*P < 0.01. tP < 0.05. more than one offspring in a family, one of these was randomly selected for inclusion in the analyses. The results of this procedure are shown in Table 3. As can be seen from this table, the effect of mating pairs is significant for both hand and ear preference while there is no effect for eye or foot preference. Thus the total pattern of this analysis is consistent with the correlational findings. The present data suggest that at least two of the indices of lateral preference (hand and ear) have causal components that can be traced to the common factors shared by family members. However, if there were a common underlying causative process, one would expect that the patterns of familial resemblance would be similar for all four indices of lateral preference. This does not appear to be the case since handedness and earedness show relationships which eye and foot preference do not. Individual parrems of laferulity
If there is a common factor underlying all indices of lateral preference, one would expect certain conditions to hold. For instance, if an individual were right-handed, the probability that he would also be right-footed, right-eyed and right-eared would be higher than if he were left-handed. This expectation suggests that there should be significant correlations between the four indices of laterality when measured within individuals. To assess the degree of relationship among the various forms of laterality, data from the entire group of 701 individuals were used. The laterality scores were computed as they were for the correlational analysis of familial resemblance. Then intercorrelations were generated among the four measures. These results are shown in Table 4. All of the paired correlations are significant with P < 0.001. Table 4. Intercorrelations
Eye
between measures of laterality* (IV = 701)
Eye
Hand
Foot
Ear
I .oo
0.29
0.25 0.48
0.19 0.23 0.29
Hand Foot Ear *All correlations with P < 0.001.
I .oo
I .oo
1.00 in this matrix are significant
The existence of significant correlations among the four indices of laterality seems to suggest that there is some common factor that underlies lateral preference. However, the sizes of the correlations indicate that the contribution of such a factor may be quite small. The largest correlation is that between handedness and footedness. This correlation is quite sizeable and accounts for 23 % of the common variance. All of the other correlations, although significant, are quite small. Each of these only accounts for between 4 and 8 % of the predicted variance. This would seem to suggest that there is a strong common mechanism determining laterality of hand and foot, while the other manifestations of laterality have only a small degree of commonality. To the extent that there is a single common mechanism underlying all forms of lateral preference, all of the
NOTE
547
obtained correlations should not only have been significant and positive but also of approximately the same magnitude. However, this is not the case for these data. The correlation between handedness and footedness is significantly greater than that found for hand and ear and hand and eye. It is also significantly greater than the correlation between foot and eye and foot and ear (all of these obtained differences are significant at P < 0.001). In addition, the correlation between eyedness and earedness seems to be weaker than the other obtained relationships [‘I]. This pattern of results seems to imply that there is a strong common factor supporting the preference of hand and foot that is not shared by the other dimensions of laterality. One can explain the strength of the relationship between handedness and footedness. The neural control of a limb seems to rest primarily with the cerebral hemisphere that is contralateral to it. Hence, the correlation between handedness and footedness may reflect the involvement of a dominant cerebral hemisphere. It is more difficult to implicate this mechanism in the formation of the laterality of the eye and the ear since each eye and ear is bilaterally represented in the cortex. These data suggest several answers to questions concerning the mechanisms which support lateral preference in humans. There is some suggestion that the lateral preferences of the hand and of the ear have a familial component, although whether or not this component is genetic remains unresolved. Furthermore, the asymmetrical nature of the relationships amongst the various indices of laterality, in conjunction with the different patterns of familial resemblances among the measures, suggests that there is probably more than one mechanism that is responsible for the development and the maintenance of lateral preference behaviors. AcA-truwler/~ettlenrs-We acknowledge the assistance of R. CROPP and J. PORAC and the financial assistance of the Natural Sciences and Engineering Research Council of Canada and the Medical Research Council of Canada. This work represents an equal and shared contribution of both authors.
REFERENCES eye. Psychol. Bull. 83. 880-897, 1976. I. PORAC, C. and COREN, S. The dominant Latetnli/.s characteristics and their edrtcatiotral ittrplications. University 2. CLARK, M. M. Left-handedness: of London Press, London, 1957. between lateral preference behaviors in humans. Behav. Bruin 3. PORAC, C. and COREN, S. Relationships Sri. 2, 31 l-312, 1978. pp. 4. LEVY, J. In Hettrisphere /rrtrctiotl in fhe hutt~atr brain, S. J. DI~IOND and J. G. BEAU~~ONT (Editors), 121-183. Elek Science, London, 1974. basis of human laterality. Behav. Bruin Sri. 2, 5. COREALLIS, M. C. and MORGAN, M. J. On the biological 261, 1978. in the nervous system, S. HARSARD, R. W. DOTY, J. JAYIVES, L. GOLD6. MORGAN, M. J. In Lafetuli:ation STEINand G. KRAUTHAMER (Editors), pp. 173-194. Academic Press, New York, 1977. of handedness and cerebral dominance. Nafllre 204, 59-60, 1964. 7. ANNETT, M. A model of the inheritance in families. Ann. Huttr. Gener., Lond. 37, 93-105, 1973. 8. ANSETT, M. Handedness influences on handedness. Behav. Gene{. 8, 227-249, 1978. 9. ASSETT, M. Genetic and non-genetic Gener. 72, 117-128. 1972. 10. LEVY, J. and NAGYLAKI, T. A model for the genetics of handedness. Association, New York, 1946. I I. Blau, A. Tire Master Hand. American Orthopsychiatric mice live in right-handed worlds. Science 187, 181-184, 1975. 12. COLLISS, R. L. When left-handed in the trervous system, S HARNARD, R W. DOTY, J. JAYNES, L. GOLD13. COLLINS,R. L. In Lateralization STEINand G. KRAUTHAMER (Editors), pp. 137-150. Academic Press, New York, 1977. items for the measurement of lateral 14. COREN, S. and PORAC, C. The validity and reliability of self-report preference. Br. J. Psychol. 55, 208-213, 1978. questionaire 15. RACZKOWSKI, D., KALAT, J. W. and NEBES, R. Reliability and validity of some handedness items. Newopsc~~hologia 12, 43-48, 1974. of lateral preference. Paper presented at the 16. PORAC. C. and CORES, S. Age trends in manifestations meetings of the Canadian Psychological Association, Toronto, Ontario, June 1976. behaviors a manifestation of a single underlying process? 17. CORES, S. and PORAC. C. Are laterality Paper presented at the meetings of the Canadian Psychological Association, Ottawa, Ontario, June 1978. 18. FALCOSER, D. S. Introduction to Qltantita/ive Genetics. Ronald Press, New York, 1972. W. H. Freeman, San Francisco, 19. MCCLEARN, G. E. and DE FRIES,J. C. Inrroducrion IO BehavioralGenetics. 1973. a family study. Am. J. Hum. Gene,. 11, 52-62, 1959. 20. FALEK, A. Handedness: J. L. and KINTZ, B. L. Cotnputational Hundbook of Sfatisfics. Scott Foresman. Glenview, 21. Bruning, Illinois, 1968.
NOTE
0" a etabli la lateralit du pied, de la main, de l'oeil et de l'oreille chez des individus biologiquement relies. bien que des ressemblances v&s
entre les meres et leurs descendants
ont BtB trou-
pour la main et l'oreille,
le type des corrOlations ne se conforme pas a celui qu'on attendrait s'il s'agissait de determination
gln@tique.
Les 4 indices de lateralit
dant, la diffgrence
semble Etre correles, cepen-
dans le degr6 d'association
entre les formes de
la lat6ralit6 suggere qu'il est improbable qu'un mdcanisme causal unique rende compte de l'ensemble des manifestations t&ale, membres et organes des sens.
de la prefgrence
la-
Deutschsprachige Zusammenfassung: Seitendominanz von Hand, FUS, Auge und Ohr wurde bestimmt bei biologisch verwandten Individuen. Obwohl Ahnlichkeiten mit der Mutter beziiglichHand- und Ohrdominanz gefunden wurden, entspricht das Korrelationsmuster nicht dem aufgrund genetischer Determinierung erwarteten. Alle vier Seitigkeitsindices scheinen zu interkorrelieren; das AusmaB der Verbindung der einzelnen Lateralitatsformen deutet allerdings darauf hin, daR ein einziger kausaler Mechanismus bei der Manifestation von Clied- und Sinnesorgan-Seitenpraferenzen unwahrscheinlich ist.