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Handedness and Season of Birth: A Gender-Invariant Relation
NOTE HANDEDNESS AND SEASON OF BIRTH: A GENDER-INVARIANT RELATION Maryanne Martin1 and Gregory V. Jones2 (1University of Oxford, Oxford, U.K.; 2University of Warwick, Coventry, U.K.)
ABSTRACT In an earlier study, Dellatolas, Curt and Lellouch (1991) concluded that handedness is not related to season of birth. However, post-hoc exploration of their and other sets of data has shown that there is an apparent tendency for left-handedness to be more prevalent in the period March-July than in the period August-February. The present work tested this seasonal hypothesis prospectively among university students. It was found that the proportion of all left-handed participants who were born in the period March-July was indeed significantly greater than the proportion of all right-handed participants who were born in the same period. Furthermore, the pattern of seasonal influence upon handedness did not vary significantly between females and males. The relation between handedness and season of birth may be linked to seasonal variation in other factors such as the incidence of infectious agents. Key words: handedness, season of birth, gender, left-handed, right-handed INTRODUCTION Does the likelihood that a person will be left-handed depend on the time of year at which that person is born? In an important study of handedness among males conscripted to the French army, Dellatolas, Curt and Lellouch (1991) reported that it does not. They concluded that “no clearly significant relationship between handedness and… season of birth was found” (Dellatolas et al., 1991, p. 139), with the article’s title itself stating that handedness and month of birth “are not related”. Nevertheless, as the authors noted, the incidence of left-handedness was numerically (though not significantly) greater in the spring and summer half-year (March to August) than in the autumn and winter half-year (September to February). The division of the year into equal halves by Dellatolas et al., though procedurally neat, was not dictated by theoretical necessity. In a subsequent re-analysis, Rogerson (1994) introduced unequal divisions, examining all twelve possible groupings of five and seven consecutive months. In addition to the data of Dellatolas et al., Rogerson included two further sets of data, one relating to males aged 18-42 years and extracted from information collected by the U.S. Bureau of the Census and the National Center for Health Statistics (Inter-University Consortium for Political and Social Research, 1990), and the other extracted from records for male American baseball players (Neft and Cohen, 1991). When the three sets of data were pooled, Rogerson found that the optimal differentiation between high and low rates of left-handedness occurred for the groupings March-July versus AugustFebruary: 41.2% of all left-handed people were born in the five months March-July, but only 38.2% of all right-handed people. It should be noted that comparison between proportions in this way constitutes a procedure which is sensitive only to the possible influence of date of birth. Thus the fact that left-handed people are considerably overCortex, (1999) 35, 123-128
124
Maryanne Martin and Gregory V. Jones
represented among baseball players meant that the absolute levels of handedness were not representative of the general population. However, the relative proportion of those born within one part of the year rather than another was orthogonal to this factor, and the enhanced total number in the left-handed group served only to increase the statistical power of the comparison with respect to the more numerous right-handed group. Rogerson reported that the difference between the two aggregate proportions yielded a nominal value for χ2 (1) of 10.03, but noted that it was not possible to attach the normal tabulated probability level to this value because the relevant grouping (namely, March-July vs. August-February) was selected only post hoc from all the possible consecutive groupings of five months versus seven months. This means that the true probability that the underlying distribution is uniform is substantially higher than the tabulated level (in other words, that the true significance of any deviation from uniformity is substantially weaker than the tabulated level). The evidence from these data-sets of a relationship between handedness and season of birth, being based on a post-hoc rather than an a-priori grouping of months, is thus suggestive rather than conclusive. To overcome this problem, we report here a prospective rather than a retrospective test of the hypothesis that the proportion of left-handed people who were born in the five months March-July is greater than the proportion of right-handed people who were born in this period. In addition, it is noteworthy that Dellatolas et al. (1991) and Rogerson (1994) collected data relating only to males. It is well-established that the incidence of left-handedness in the population is somewhat lower in females than in males (e.g., Corballis, 1997). Thus it is possible that males and females differ also in their patterns of dependence (if any) of handedness upon season of birth. The present study gathered data from both females and males, allowing this possibility also to be tested. MATERIALS
AND
METHODS
There were 772 participants, of whom 395 were female and 377 were male. The participants were recruited within a programme of experiments investigating the influence of handedness upon memory for orientation (e.g., Jones and Martin, 1997; Martin and Jones, 1998). Recruitment was directed at obtaining a relatively high proportion of left-handed participants, and approximately equal proportions of female and male participants. No information on date of birth was known prior to recruitment. All those recruited who were students at the University of Oxford born in the period 1972-1978 inclusive (and thus aged between 18 and 25 years) were entered into the present study. Handedness was assessed as both a binary and a graded measure. For the binary measure, participants responded to the following: “Would you describe yourself as lefthanded or right-handed? In particular, which hand do you use for drawing?” For the graded measure, participants completed the handedness questionnaire of Annett (1985). This elicits handedness preferences for each of twelve different activities (e.g., throwing a ball; holding a toothbrush). Right, equal, and left responses are assigned scores of 1, 2 and 3, respectively, resulting in overall scores ranging from 12 (complete right) to 36 (complete left). RESULTS The results were first analysed using the binary measure of handedness. Table I shows the frequencies of left-handed participants and of right-handed participants who were born in each month. Initial analysis showed that the patterns of the two distributions over the twelve months differed significantly from each other, c2 (11, N = 772) = 24.81, p < .01. The data were further analysed to examine whether or not they conformed to the hypothesis that a higher proportion of all left-handed than of all right-handed participants were born in the five months March-July. Table II shows the data grouped over months. Analysis confirmed that the patterns of the two handedness distributions over the two groups of months differed significantly from
Handedness and season of birth
125
TABLE I
Distribution of Handedness (Binary and Graded Measures) over Twelve Months of Birth Frequency
Percentage
LH
RH
LH
RH
Mean handedness
18 17 17 23 24 19 22 9 20 22 21 27
29 30 37 36 37 40 28 53 60 82 54 47
7.5 7.1 7.1 9.6 10.0 7.9 9.2 3.8 8.4 9.2 8.8 11.3
5.4 5.6 6.9 6.8 6.9 7.5 5.3 9.9 11.3 15.4 10.1 8.8
20.2 19.7 20.0 20.5 19.8 19.1 20.9 16.0 18.0 17.2 18.4 19.6
Month January February March April May June July August September October November December
Note: LH and RH refer to left-handed and right-handed participants, respectively (binary classification). Mean handedness refers to mean Annett score (graded classification).
each other, χ2 (1, N = 772) = 7.89, p < .01. It was also investigated whether the results differed between female and male participants. The null hypothesis was that the frequencies in corresponding cells of the female and male sub-tables in Table II were in the same ratio as that between the total numbers of female and male participants (i.e., 1.048:1). The empirical deviation from this null hypothesis was found not to be significant, χ2 (1, N = 772) = 0.66, indicating that the effect of season did not vary significantly with gender. The categorization of birth months into March-July versus August-February was dictated by a-priori considerations. The effects of alternative post-hoc categorizations may also be explored although, in these cases, tabulated levels of significance cannot be attached to the outcomes of multiple nonindependent analyses. When the patterns of handedness associated with all possible categorizations of five and seven consecutive months were examined (for all participants), it was found that a value of χ2 (1, N = 772) numerically higher than that for the a-priori categorization was obtained only for the groupings January-July versus August-December (χ2 = 13.15) and December-June versus July-November (χ2 = 10.56). The precise category boundaries which are associated with maximal differentiation are of TABLE II
Distribution of Handedness over Two Groups of Months of Birth Frequency
Percentage
Group of months
LH
RH
LH
RH
March-July August-February
105 134
All participants 178 355
43.9 56.1
33.4 66.6
March-July August-February
51 72
Female participants 90 182
41.5 58.5
33.1 66.9
March-July August-February
54 62
Male participants 88 173
46.6 53.4
33.7 66.3
Note: LH = left-handed, RH = right-handed.
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Maryanne Martin and Gregory V. Jones
course inevitably subject to some degree of random variation in individual studies. Nevertheless, it can be seen that there is substantial overlap between the a-priori and posthoc categorizations, with each of them including the segments March-June and AugustNovember. The results were investigated further using the graded measure of handedness. An analysis of variance was carried out to investigate whether the mean level of Annett handedness score for each of the twelve months (shown in Table I) varied with month, and also with gender. There was a significant effect of month, F (11, 720) = 1.93, p < .05, but no significant effect of gender, F (1, 720) < 1, or interaction between gender and month, F (11, 720) < 1 (the number of participants was slightly lower than in previous analyses because a few participants did not receive the Annett questionnaire). A further analysis was carried out on Annett handedness scores with the months combined into the two a-priori groups March-July and August-February. The results were similar in that there was a significant effect of month group, F (1, 740) = 8.14, p < .01, but there was no significant effect of gender or interaction between gender and month group, both F (1, 740) < 1; the mean Annett handedness score was higher (i.e., there was more left-handedness) for MarchJuly than for August-February, at 20.0 and 18.2, respectively. All the preceding analyses examined directly the hypothesis that season of birth influences handedness. It is also of interest, however, to investigate briefly a complementary issue, that of the extent to which a person’s handedness conveys information about the person’s season of birth. To address this issue, season of birth (birth in March-July and in August-February were scored as 1 and 0, respectively) was regressed on Annett handedness score, for all participants. Because of the dichotomous nature of season of birth, logistic regression analysis was employed. This showed that there was a significant relation between the two variables, χ2 (1, N = 744) = 7.98, p < .01. The best-fitting equation for the logit function, L, was L = 0.0251α – 1.025, where α represents Annett handedness score. Now L = ln [π/(1 — π)], where π represents the probability of being born in the season MarchJuly, and hence, reversing this transformation, π = 1/[1 + exp (– L)]. When α took values of 12 (extreme right) and of 36 (extreme left), the predicted values of L were – 0.724 and – 0.121, respectively, thus corresponding to predicted values for π of 0.327 and 0.470, respectively. DISCUSSION There have been a number of investigations of the possible dependency of human handedness upon season of birth. Although early studies produced conflicting reports (e.g., Badian, 1983; Leviton and Kilty, 1979), recent post-hoc analysis of three large data-sets (Dellatolas et al. 1991; Rogerson, 1994) has suggested that people born during the five months March-July are more likely to be left-handed than those born outside this period. The important question scientifically is whether this hypothesis is successful predictively as well as descriptively. The present results confirm that this is the case. As predicted, the proportion of left-handed students who were born during the period March-July, 43.9%, was found to be significantly higher than the corresponding proportion of right-handed students, 33.4%. The present results also provide evidence concerning the possible role of gender. Only males were included in the three previous data-sets, but both females and males were included in the present study. The relation between handedness and season of birth was found here to be gender-invariant. That is, the pattern of seasonal influence upon handedness was found not to vary significantly between females and males. The magnitude of the seasonal effect in the present study was somewhat greater than in the previous studies. This is likely to have resulted from a difference in sampling strategy. The subjects of the present study comprised students from a limited birth cohort (a range of seven years) at a university which draws most of its intake from a relatively homogeneous population. In contrast, the subjects in the previous studies were drawn from more heterogeneous sources. The subjects in the two groups of Rogerson (1994) both had a much larger age range of approximately 25 years. The subjects of Dellatolas et al. (1991) possessed an age range similar to that in the present study, but were drawn otherwise from
Handedness and season of birth
127
an unrestricted population (Dellatolas et al. stated that all French men of the appropriate age were subject to army conscription). It is well-established that powerful secular, social and cultural influences may be exerted upon the incidence of left-handedness (see Harris, 1990), and there is considerable evidence also that patterns of handedness may differ as a function of degree of intellectual giftedness (see Szeszko, Madden and Piro, 1997). By minimising the degree of extraneous variance derived from such influences, the present study will have allowed the effect of season of birth to emerge relatively distinctly. Why should those born between March and July comprise a higher proportion of all left-handed people than of all right-handed people? A number of factors such as patterns of disease and of nutrition vary with the seasons and thus can affect development both before and after birth. It is noteworthy that two forms of disorder which appear to be linked in some degree to handedness both themselves exhibit seasonal dependency. The first of these is developmental dyslexia, where meta-analysis of the results of a large number of studies indicates that there is a small but reliable association between left-handedness and reading difficulties in childhood (see Martin and Jones, in press). It has been reported that developmental dyslexia is more widespread among children born during the period MayJuly, with the increase attributed to greater exposure to infectious agents (e.g., measles and influenza) when the second trimester of pregnancy occurs in the winter (Livingston, Adam and Bracha, 1993). The second type of disorder is schizophrenia, where evidence exists of an association with some reversal of predominant patterns of lateralization, including handedness (e.g., Crow, Done and Sacker, 1996; DeLisi, Sakuma, Kushner et al., 1997). There is also widespread evidence of a relation between season of birth and incidence of schizophrenia; for example, Verdoux, Takei, de Saint-Mathurin et al. (1997) reported that the incidence peaks for April births (with an elevation of 13%). A recent review (Torrey, Miller, Rawlings et al., 1997) concluded, however, that it is not at present possible to decide among several possible factors which may be responsible for the relation (namely, infectious agents; temperature and weather; ambient light and internal chemistry; nutrition; toxins; complications in pregnancy and birth; and genetic effects). A range of factors as wide as this must also be considered in attempting to reach a satisfactory understanding of the relation between season of birth and handedness. However, we have here reached the stage only of demonstrating that a significant relation does in fact exist. It is hoped that this finding will encourage further work to dissect out the factor or factors which underlie the observed relation between season of birth and handedness. REFERENCES ANNETT, M. Left, Right, Hand and Brain: The Right Shift Theory. London: Erlbaum, 1985. BADIAN, N.A. Birth order, maternal age, season of birth, and handedness. Cortex, 19: 451-463, 1983. CORBALLIS, M.C. The genetics and evolution of handedness. Psychological Review, 104: 714-727, 1997. CROW, T.J., DONE, D.J., and SACKER, A. Cerebral lateralization is delayed in children who later develop schizophrenia. Schizophrenia Research, 22: 181-185, 1996. DELISI, L.E., SAKUMA, M., KUSHNER, M., FINER, D.L., HOFF, A.L., and CROW, T.J. Anomalous cerebral asymmetry and language processing in schizophrenia. Schizophrenia Bulletin, 23: 255-271, 1997. DELLATOLAS , G., CURT, F., and LELLOUCH, J. Birth order and month of birth are not related with handedness in a sample of 9370 young men. Cortex, 27: 137-140, 1991. HARRIS, L.J. Cultural influences on handedness: Historical and contemporary theory and evidence. In S. Coren (Ed.), Left-handedness: Implications and Anomalies. Amsterdam: North-Holland, 1990, pp. 195-258. INTER-UNIVERSITY CONSORTIUM FOR POLITICAL AND SOCIAL RESEARCH. National Health and Nutrition Examination Survey II, 1976-1980: Anthropometric Data. Ann Arbor, Michigan: Inter-university Consortium for Political and Social Research, 1990. JONES, G.V., and MARTIN, M. Handedness dependency in recall from everyday memory. British Journal of Psychology, 88: 609-619, 1997. LEVITON, A., and KILTY, T. Seasonal variation in the birth of left-handed schoolgirls. Archives of Neurology, 36: 115-116, 1979. LIVINGSTON, R., ADAM, B.S., and BRACHA, H.S. Season of birth and neurodevelopment disorders: Summer birth is associated with dyslexia. Journal of the American Academy of Child and Adolescent Psychiatry, 32: 612-616, 1993.
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MARTIN, M., and JONES, G.V. Generalizing everyday memory: Signs and handedness. Memory & Cognition, 26: 193-200, 1998. MARTIN, M., and JONES, G.V. Motor imagery theory of a contralateral handedness effect in recognition memory: Towards a chiral psychology of cognition. Journal of Experimental Psychology: General, in press. NEFT, D., and COHEN, R. The Sports Encyclopedia. New York: St. Martin’s Press, 1991. ROGERSON, P.A. On the relationship between handedness and season of birth for men. Perceptual and Motor Skills, 79: 499-506, 1994. SZESZKO, P.R., MADDEN, G.M., and PIRO, J.M. Factor analyses of handedness items in left and righthanded intellectually gifted and nongifted children. Cortex, 33: 579-584, 1997. TORREY, E.F., MILLER, J., RAWLINGS, R., and YOLKEN, R.H. Seasonality of births in schizophrenia and bipolar disorder: A review of the literature. Schizophrenia Research, 28: 1-38, 1997. VERDOUX, H., TAKEI, N., DE SAINT-MATHURIN, R.C., and BOURGEOIS, M. Analysis of the seasonal variation of schizophrenic births using a Kolmogorov-Smirnov type statistic. European Psychiatry, 12: 111-116, 1997. Dr. Maryanne Martin, Department of Experimental Psychology, University of Oxford, South Parks Road, Oxford OX1 3UD, U.K.; e-mail [email protected]
(Received 4 June 1998; accepted 20 August 1998)
ABSTRACT In an earlier study, Dellatolas, Curt and Lellouch (1991) concluded that handedness is not related to season of birth. However, post-hoc exploration of their and other sets of data has shown that there is an apparent tendency for left-handedness to be more prevalent in the period March-July than in the period August-February. The present work tested this seasonal hypothesis prospectively among university students. It was found that the proportion of all left-handed participants who were born in the period March-July was indeed significantly greater than the proportion of all right-handed participants who were born in the same period. Furthermore, the pattern of seasonal influence upon handedness did not vary significantly between females and males. The relation between handedness and season of birth may be linked to seasonal variation in other factors such as the incidence of infectious agents. Key words: handedness, season of birth, gender, left-handed, right-handed INTRODUCTION Does the likelihood that a person will be left-handed depend on the time of year at which that person is born? In an important study of handedness among males conscripted to the French army, Dellatolas, Curt and Lellouch (1991) reported that it does not. They concluded that “no clearly significant relationship between handedness and… season of birth was found” (Dellatolas et al., 1991, p. 139), with the article’s title itself stating that handedness and month of birth “are not related”. Nevertheless, as the authors noted, the incidence of left-handedness was numerically (though not significantly) greater in the spring and summer half-year (March to August) than in the autumn and winter half-year (September to February). The division of the year into equal halves by Dellatolas et al., though procedurally neat, was not dictated by theoretical necessity. In a subsequent re-analysis, Rogerson (1994) introduced unequal divisions, examining all twelve possible groupings of five and seven consecutive months. In addition to the data of Dellatolas et al., Rogerson included two further sets of data, one relating to males aged 18-42 years and extracted from information collected by the U.S. Bureau of the Census and the National Center for Health Statistics (Inter-University Consortium for Political and Social Research, 1990), and the other extracted from records for male American baseball players (Neft and Cohen, 1991). When the three sets of data were pooled, Rogerson found that the optimal differentiation between high and low rates of left-handedness occurred for the groupings March-July versus AugustFebruary: 41.2% of all left-handed people were born in the five months March-July, but only 38.2% of all right-handed people. It should be noted that comparison between proportions in this way constitutes a procedure which is sensitive only to the possible influence of date of birth. Thus the fact that left-handed people are considerably overCortex, (1999) 35, 123-128
124
Maryanne Martin and Gregory V. Jones
represented among baseball players meant that the absolute levels of handedness were not representative of the general population. However, the relative proportion of those born within one part of the year rather than another was orthogonal to this factor, and the enhanced total number in the left-handed group served only to increase the statistical power of the comparison with respect to the more numerous right-handed group. Rogerson reported that the difference between the two aggregate proportions yielded a nominal value for χ2 (1) of 10.03, but noted that it was not possible to attach the normal tabulated probability level to this value because the relevant grouping (namely, March-July vs. August-February) was selected only post hoc from all the possible consecutive groupings of five months versus seven months. This means that the true probability that the underlying distribution is uniform is substantially higher than the tabulated level (in other words, that the true significance of any deviation from uniformity is substantially weaker than the tabulated level). The evidence from these data-sets of a relationship between handedness and season of birth, being based on a post-hoc rather than an a-priori grouping of months, is thus suggestive rather than conclusive. To overcome this problem, we report here a prospective rather than a retrospective test of the hypothesis that the proportion of left-handed people who were born in the five months March-July is greater than the proportion of right-handed people who were born in this period. In addition, it is noteworthy that Dellatolas et al. (1991) and Rogerson (1994) collected data relating only to males. It is well-established that the incidence of left-handedness in the population is somewhat lower in females than in males (e.g., Corballis, 1997). Thus it is possible that males and females differ also in their patterns of dependence (if any) of handedness upon season of birth. The present study gathered data from both females and males, allowing this possibility also to be tested. MATERIALS
AND
METHODS
There were 772 participants, of whom 395 were female and 377 were male. The participants were recruited within a programme of experiments investigating the influence of handedness upon memory for orientation (e.g., Jones and Martin, 1997; Martin and Jones, 1998). Recruitment was directed at obtaining a relatively high proportion of left-handed participants, and approximately equal proportions of female and male participants. No information on date of birth was known prior to recruitment. All those recruited who were students at the University of Oxford born in the period 1972-1978 inclusive (and thus aged between 18 and 25 years) were entered into the present study. Handedness was assessed as both a binary and a graded measure. For the binary measure, participants responded to the following: “Would you describe yourself as lefthanded or right-handed? In particular, which hand do you use for drawing?” For the graded measure, participants completed the handedness questionnaire of Annett (1985). This elicits handedness preferences for each of twelve different activities (e.g., throwing a ball; holding a toothbrush). Right, equal, and left responses are assigned scores of 1, 2 and 3, respectively, resulting in overall scores ranging from 12 (complete right) to 36 (complete left). RESULTS The results were first analysed using the binary measure of handedness. Table I shows the frequencies of left-handed participants and of right-handed participants who were born in each month. Initial analysis showed that the patterns of the two distributions over the twelve months differed significantly from each other, c2 (11, N = 772) = 24.81, p < .01. The data were further analysed to examine whether or not they conformed to the hypothesis that a higher proportion of all left-handed than of all right-handed participants were born in the five months March-July. Table II shows the data grouped over months. Analysis confirmed that the patterns of the two handedness distributions over the two groups of months differed significantly from
Handedness and season of birth
125
TABLE I
Distribution of Handedness (Binary and Graded Measures) over Twelve Months of Birth Frequency
Percentage
LH
RH
LH
RH
Mean handedness
18 17 17 23 24 19 22 9 20 22 21 27
29 30 37 36 37 40 28 53 60 82 54 47
7.5 7.1 7.1 9.6 10.0 7.9 9.2 3.8 8.4 9.2 8.8 11.3
5.4 5.6 6.9 6.8 6.9 7.5 5.3 9.9 11.3 15.4 10.1 8.8
20.2 19.7 20.0 20.5 19.8 19.1 20.9 16.0 18.0 17.2 18.4 19.6
Month January February March April May June July August September October November December
Note: LH and RH refer to left-handed and right-handed participants, respectively (binary classification). Mean handedness refers to mean Annett score (graded classification).
each other, χ2 (1, N = 772) = 7.89, p < .01. It was also investigated whether the results differed between female and male participants. The null hypothesis was that the frequencies in corresponding cells of the female and male sub-tables in Table II were in the same ratio as that between the total numbers of female and male participants (i.e., 1.048:1). The empirical deviation from this null hypothesis was found not to be significant, χ2 (1, N = 772) = 0.66, indicating that the effect of season did not vary significantly with gender. The categorization of birth months into March-July versus August-February was dictated by a-priori considerations. The effects of alternative post-hoc categorizations may also be explored although, in these cases, tabulated levels of significance cannot be attached to the outcomes of multiple nonindependent analyses. When the patterns of handedness associated with all possible categorizations of five and seven consecutive months were examined (for all participants), it was found that a value of χ2 (1, N = 772) numerically higher than that for the a-priori categorization was obtained only for the groupings January-July versus August-December (χ2 = 13.15) and December-June versus July-November (χ2 = 10.56). The precise category boundaries which are associated with maximal differentiation are of TABLE II
Distribution of Handedness over Two Groups of Months of Birth Frequency
Percentage
Group of months
LH
RH
LH
RH
March-July August-February
105 134
All participants 178 355
43.9 56.1
33.4 66.6
March-July August-February
51 72
Female participants 90 182
41.5 58.5
33.1 66.9
March-July August-February
54 62
Male participants 88 173
46.6 53.4
33.7 66.3
Note: LH = left-handed, RH = right-handed.
126
Maryanne Martin and Gregory V. Jones
course inevitably subject to some degree of random variation in individual studies. Nevertheless, it can be seen that there is substantial overlap between the a-priori and posthoc categorizations, with each of them including the segments March-June and AugustNovember. The results were investigated further using the graded measure of handedness. An analysis of variance was carried out to investigate whether the mean level of Annett handedness score for each of the twelve months (shown in Table I) varied with month, and also with gender. There was a significant effect of month, F (11, 720) = 1.93, p < .05, but no significant effect of gender, F (1, 720) < 1, or interaction between gender and month, F (11, 720) < 1 (the number of participants was slightly lower than in previous analyses because a few participants did not receive the Annett questionnaire). A further analysis was carried out on Annett handedness scores with the months combined into the two a-priori groups March-July and August-February. The results were similar in that there was a significant effect of month group, F (1, 740) = 8.14, p < .01, but there was no significant effect of gender or interaction between gender and month group, both F (1, 740) < 1; the mean Annett handedness score was higher (i.e., there was more left-handedness) for MarchJuly than for August-February, at 20.0 and 18.2, respectively. All the preceding analyses examined directly the hypothesis that season of birth influences handedness. It is also of interest, however, to investigate briefly a complementary issue, that of the extent to which a person’s handedness conveys information about the person’s season of birth. To address this issue, season of birth (birth in March-July and in August-February were scored as 1 and 0, respectively) was regressed on Annett handedness score, for all participants. Because of the dichotomous nature of season of birth, logistic regression analysis was employed. This showed that there was a significant relation between the two variables, χ2 (1, N = 744) = 7.98, p < .01. The best-fitting equation for the logit function, L, was L = 0.0251α – 1.025, where α represents Annett handedness score. Now L = ln [π/(1 — π)], where π represents the probability of being born in the season MarchJuly, and hence, reversing this transformation, π = 1/[1 + exp (– L)]. When α took values of 12 (extreme right) and of 36 (extreme left), the predicted values of L were – 0.724 and – 0.121, respectively, thus corresponding to predicted values for π of 0.327 and 0.470, respectively. DISCUSSION There have been a number of investigations of the possible dependency of human handedness upon season of birth. Although early studies produced conflicting reports (e.g., Badian, 1983; Leviton and Kilty, 1979), recent post-hoc analysis of three large data-sets (Dellatolas et al. 1991; Rogerson, 1994) has suggested that people born during the five months March-July are more likely to be left-handed than those born outside this period. The important question scientifically is whether this hypothesis is successful predictively as well as descriptively. The present results confirm that this is the case. As predicted, the proportion of left-handed students who were born during the period March-July, 43.9%, was found to be significantly higher than the corresponding proportion of right-handed students, 33.4%. The present results also provide evidence concerning the possible role of gender. Only males were included in the three previous data-sets, but both females and males were included in the present study. The relation between handedness and season of birth was found here to be gender-invariant. That is, the pattern of seasonal influence upon handedness was found not to vary significantly between females and males. The magnitude of the seasonal effect in the present study was somewhat greater than in the previous studies. This is likely to have resulted from a difference in sampling strategy. The subjects of the present study comprised students from a limited birth cohort (a range of seven years) at a university which draws most of its intake from a relatively homogeneous population. In contrast, the subjects in the previous studies were drawn from more heterogeneous sources. The subjects in the two groups of Rogerson (1994) both had a much larger age range of approximately 25 years. The subjects of Dellatolas et al. (1991) possessed an age range similar to that in the present study, but were drawn otherwise from
Handedness and season of birth
127
an unrestricted population (Dellatolas et al. stated that all French men of the appropriate age were subject to army conscription). It is well-established that powerful secular, social and cultural influences may be exerted upon the incidence of left-handedness (see Harris, 1990), and there is considerable evidence also that patterns of handedness may differ as a function of degree of intellectual giftedness (see Szeszko, Madden and Piro, 1997). By minimising the degree of extraneous variance derived from such influences, the present study will have allowed the effect of season of birth to emerge relatively distinctly. Why should those born between March and July comprise a higher proportion of all left-handed people than of all right-handed people? A number of factors such as patterns of disease and of nutrition vary with the seasons and thus can affect development both before and after birth. It is noteworthy that two forms of disorder which appear to be linked in some degree to handedness both themselves exhibit seasonal dependency. The first of these is developmental dyslexia, where meta-analysis of the results of a large number of studies indicates that there is a small but reliable association between left-handedness and reading difficulties in childhood (see Martin and Jones, in press). It has been reported that developmental dyslexia is more widespread among children born during the period MayJuly, with the increase attributed to greater exposure to infectious agents (e.g., measles and influenza) when the second trimester of pregnancy occurs in the winter (Livingston, Adam and Bracha, 1993). The second type of disorder is schizophrenia, where evidence exists of an association with some reversal of predominant patterns of lateralization, including handedness (e.g., Crow, Done and Sacker, 1996; DeLisi, Sakuma, Kushner et al., 1997). There is also widespread evidence of a relation between season of birth and incidence of schizophrenia; for example, Verdoux, Takei, de Saint-Mathurin et al. (1997) reported that the incidence peaks for April births (with an elevation of 13%). A recent review (Torrey, Miller, Rawlings et al., 1997) concluded, however, that it is not at present possible to decide among several possible factors which may be responsible for the relation (namely, infectious agents; temperature and weather; ambient light and internal chemistry; nutrition; toxins; complications in pregnancy and birth; and genetic effects). A range of factors as wide as this must also be considered in attempting to reach a satisfactory understanding of the relation between season of birth and handedness. However, we have here reached the stage only of demonstrating that a significant relation does in fact exist. It is hoped that this finding will encourage further work to dissect out the factor or factors which underlie the observed relation between season of birth and handedness. REFERENCES ANNETT, M. Left, Right, Hand and Brain: The Right Shift Theory. London: Erlbaum, 1985. BADIAN, N.A. Birth order, maternal age, season of birth, and handedness. Cortex, 19: 451-463, 1983. CORBALLIS, M.C. The genetics and evolution of handedness. Psychological Review, 104: 714-727, 1997. CROW, T.J., DONE, D.J., and SACKER, A. Cerebral lateralization is delayed in children who later develop schizophrenia. Schizophrenia Research, 22: 181-185, 1996. DELISI, L.E., SAKUMA, M., KUSHNER, M., FINER, D.L., HOFF, A.L., and CROW, T.J. Anomalous cerebral asymmetry and language processing in schizophrenia. Schizophrenia Bulletin, 23: 255-271, 1997. DELLATOLAS , G., CURT, F., and LELLOUCH, J. Birth order and month of birth are not related with handedness in a sample of 9370 young men. Cortex, 27: 137-140, 1991. HARRIS, L.J. Cultural influences on handedness: Historical and contemporary theory and evidence. In S. Coren (Ed.), Left-handedness: Implications and Anomalies. Amsterdam: North-Holland, 1990, pp. 195-258. INTER-UNIVERSITY CONSORTIUM FOR POLITICAL AND SOCIAL RESEARCH. National Health and Nutrition Examination Survey II, 1976-1980: Anthropometric Data. Ann Arbor, Michigan: Inter-university Consortium for Political and Social Research, 1990. JONES, G.V., and MARTIN, M. Handedness dependency in recall from everyday memory. British Journal of Psychology, 88: 609-619, 1997. LEVITON, A., and KILTY, T. Seasonal variation in the birth of left-handed schoolgirls. Archives of Neurology, 36: 115-116, 1979. LIVINGSTON, R., ADAM, B.S., and BRACHA, H.S. Season of birth and neurodevelopment disorders: Summer birth is associated with dyslexia. Journal of the American Academy of Child and Adolescent Psychiatry, 32: 612-616, 1993.
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MARTIN, M., and JONES, G.V. Generalizing everyday memory: Signs and handedness. Memory & Cognition, 26: 193-200, 1998. MARTIN, M., and JONES, G.V. Motor imagery theory of a contralateral handedness effect in recognition memory: Towards a chiral psychology of cognition. Journal of Experimental Psychology: General, in press. NEFT, D., and COHEN, R. The Sports Encyclopedia. New York: St. Martin’s Press, 1991. ROGERSON, P.A. On the relationship between handedness and season of birth for men. Perceptual and Motor Skills, 79: 499-506, 1994. SZESZKO, P.R., MADDEN, G.M., and PIRO, J.M. Factor analyses of handedness items in left and righthanded intellectually gifted and nongifted children. Cortex, 33: 579-584, 1997. TORREY, E.F., MILLER, J., RAWLINGS, R., and YOLKEN, R.H. Seasonality of births in schizophrenia and bipolar disorder: A review of the literature. Schizophrenia Research, 28: 1-38, 1997. VERDOUX, H., TAKEI, N., DE SAINT-MATHURIN, R.C., and BOURGEOIS, M. Analysis of the seasonal variation of schizophrenic births using a Kolmogorov-Smirnov type statistic. European Psychiatry, 12: 111-116, 1997. Dr. Maryanne Martin, Department of Experimental Psychology, University of Oxford, South Parks Road, Oxford OX1 3UD, U.K.; e-mail [email protected]
(Received 4 June 1998; accepted 20 August 1998)