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Secular variation in handedness over ninety years
OO?Rc
3932/8l/O30459~04$02.Ml/O 1981 Perpmon Press Ltd.
NOTE SECULAR
VARIATION
IN HANDEDNESS
OVER NINETY
YEARS
C. J. BRACKEYRIDGE
Department
of Psychiatry,
University
(Received
of Melbourne.
10 December
Victoria,
Australia
1980)
Abstract--Left-handed writers of high socio-economic status born in Australia or New Zealand were found to increase from about 2 to 13”,, over the period 188&1969. The change was well represented by a symmetrical sigmoid curve which allowed the upper asymptote to be evaluated as 13.63 + 0.16”,,. The observed secular variation differed in shape and began earlier than in the United States perhaps because of differences in social status between the samples. It is suggested that differential perinatal death may operate together with cultural relaxation in promoting an increasing proportion of sinistrals with time.
EFFECTof age on handedness has been measured in many samples of children. Earlier work reviewed by JONES Cl] tended to show an increase in dextrality with age, whereas more recent studies [2,3] have failed to demonstrate a sigmficant effect. Among adults increasing age was associated with an appreciable shift towards right-handedness in both psychiatric patients and healthy persons, the percentage of sinistral writers being lowest in the oldest age groups [4]. Comparable investigations on the secular variation of handedness summarized by LEVY [S] show that the frequency ofsinistrals rose monotonically in the United States by some 9”,, over the forty year period beginning from 1932 when it was 2.2”,,. This is usually attributed to relaxation of cultural pressure to be right-handed, though BROWN and JAFFF [6] have proposed the hypothesis that cebral dominance is not a state but a continuous process that develops through life. The present study reports on the increase in the proportion of sinistrality in an Australian population sample of homogeneous social status over ninety years. The purposes were to deduce the nature of the growth process involved, derive the asymptotic proportion of left-handedness for the population and compare the results with those obtained from American investigations. THE
SUBJECTS
AND METHODS
Material for the study was obtamed by distributing a questionnaire among scholars, aged from 6 to 16 yr, of seven private Melbourne Schools. Their parents were asked to provide anonymously details of the sex, date and country of birth, and natural writing hand of their children. They were requested to record the birth rank of all live births, together with still-births and miscarriages, and to exclude adopted children. In addition the same information was provided for the mother, father and four grandparents. Handedness was recorded as left, right and ambidextrous; however, because the last category could not always be distinguished from indefinite responses, only left and right-handed persons were included. Parents were financially advantaged with respect to the rest of the community and comprised a reasonably homogeneous sample of high socio-economic status. While there was considerable ethnic variation, only persons born in Austraha or New Zealand were used in the investigation. If the writing hand had been changed because of cultural pressure or injury, the original preference was requested. Justification of a binomial variable (left or right) to represent handedness has been given by ANNETT 173. To relate the proportion of left-handers to the year of their birth, the three-parameter logistic curve was used; this has the additional function ofallowing the upper asymptote to be estimated from the same observations as the slope and intercept. The procedure described by HCWLFTT and GOSTICK [8] was adopted to obtain the best fit to the data and the weighted regression method of mlnimum logit xz [9] was used for estimation and testing goodness of fit. 459
460
Differentiation
NOTE
of the logistic curve yields the equation dq dr =/Ml-Y)
1 dY dInY ;dr=~drm=p(l-i.)
or
Thus the rate of change ofthe logarithm of the proportion of left-handers varies directly with the proportion of righthanders in the population at the time (I). In the present context,y has been scaled to the upper limit of growth (YO)so that the proportionate ratio p=y/yO becomes the dependent variable in the logisticequation after division by (1 -p) and taking natural logarithms: In L 1-P the parameters YO.
requiring
estimation
being the intercept
= u+ht, a, the linear regression
coefficient band the upper asymptote
RESULTS Persons born between 1880 and 1969 were divided into seven groups and the percentage of left-handers was related to the mean year of birth as shown in Fig. 1. There is a monotonic increase from 2% in the last century to about 13.2% shortly before 1970. The symmetrical sigmoid shape of the curve implies that a logistic transformation of the dependent variable should vary in a linear manner with the year of birth. This was found to be so when a weighted least-squares analysis of the data was performed according to BERKSON[9]. which involved estimation of the three parameters: the slope (b), the intercept (a) and the upper asymptote (yO) together with a test of the goodness of fit of the transformed data to a straight line. The estimates obtained were b=0.0738~0.0021, a= - 1.236+0.064”,;, and y,,= 13.63~0.16’~. When backtransformed the best fitting curve is shown by the broken line in Fig. 1. Its acceptability is reflected in the logit 2’ value for goodness of fit of 9.00 (P> 0.1) indicating that there is no significant heterogeneity in the departures of the observed percentages from the fitted curve.
__---
o,_-= ,’
, ,
,’
p"
I’ ,’
I’
/’
,/’ ,'O /’ I’ I’
,d’ 1’ /’ /’ ,’
1890
1900
1910
1930
1920
YEAR
FIG. I.
1940
1950
1960
0
NOTE
461
DISCUSSION The symmetrical sigmoid curve describing the change in the proportion of sinistral Australians and New Zealanders over the past ninety years represents a sixfold increase which has practically levelled off by 1970. The growth process involved is that, at a given time, the rate of change of the logarithm of the proportion of left-handers in the population varies directly with the proportion of right-handers. The curve presented in Fig. 1 differs in several respects from that drawn by LEVY [S] from samples obtained in the United States. Both curves begin at about 29; but, whereas the process begins soon after 1930 in America, it has already begun by 1900 in Australasia. Both curves approach asymptotic levels by 1970, but the American data follow a parabolic, rather than a sigmoid, path. This suggests that the processes may vary in kind as well as degree in the two countries. On the other hand, the differences may reflect a disparity in social status. Thus if this is higher in the Australasian material and gives rise to a more tolerant attitude towards sinistrality, the process ofcultural relaxation should begin earlier than in a society of lower average socio-economic status. The argument depends upon a direct relationship between social class and tolerance towards sinistral writers. Such a correlation could be difficult to demonstrate if it is subject to secular variation. The present study cannot determine whether cultural relaxation is the only mechanism responsible for the large increase in left-handed writers over the period examined. Pathological left-handedness, the concept that such conditions as anoxia associated with complications of pregnancy [lo] produce an excess of left-handedness, is unlikely to be a factor. Over the past century, antenatal care and obstetric skills have greatly improved so that any effects of birth stress on handedness are likely to have operated in the opposite manner to the observed influence of secular variation. However, if birth complications associated with left handedness were regarded as potentially fatal, so that perinatal death took a greater toll of sinistral than dextral infants, the situation would be quite different. Differential wastage at the time of birth might then be a plausible mechanism acting in addition to sociocultural conditioning. Given the improved obstetric techniques and the more enlightened attitude towards left-handed writers in recent years, increased survival of sinistrals and cultural relaxation may be significant contributors to the observed secular trends.
REFERENCES 1. Jo~r:s. H. E. Dextrality as a function of age. J. esp. Ps~chol. 14, 125 143, 1931. 2. APGNETT, M. The growth of manual preference and speed. Br. J. Psycho/. 61, 545-558, 1970. 3. HARI>YCK, C., GOLIIMAU, R. and PETRINOVICFI,L. Handedness and sex, race and age. Hum. Biol. 47, 369- 375. 1975. 4. FI.I‘MIN(;FR,J. J., DALTON. R. and STANI>ACX,K. F. Age as a factor in the handedness of adults. Neurop.~~ch&girr 15,471-473, 1977. 5. LFVY, J. Psychobiological implications of bilateral asymmetry. In Hrmisphrrr Function in the Humun Brujn, S. DIMONI)and J. G. BEAUMONT(Editors). Elek, London, 1974. 6. BKOWN, J. W. and JAFFF,,J. Hypothesis on cerebral dominance. Nruropsycholoyia 13, 107 110, 1975. 7. ANNI:TT, M. Genetic and nongenetic influences on handedness. Behar. Gmet. 8, 227 249, 1978. 8. HFWLETT, P. S. and GOSTICK, K. G. The loss of weight of pyrethrin-treated flour beetles Triholiunl cusfunrum (Herbst) and its application to bioassay. Ann. appl. Biol. 43, 213--236, 1955. 9. BFRKSON, J. A statistically precise and relatively simple method of estimating the bioassay with quanta1 response, based on the logistic function. J. Am. Srarisr. As. 48, 565 599, 1953. 10. BAKAN, P., DIHH, G. and RE~I), P. Handedness and birth stress. Nrurop.~y~ho/oyiu 11, 363 366, 1973.
On a trouvd
une
pdriode
1880-1969,
la main
gauche,
Australie
de niveau
reprssentge
tant
d'dvaluer
La variation
tdt qu'aux
bien temps
que
que
courbe
l'asymptote observCe Etats
ces de conditions suggere
Zelande.
par une
Unis,
sociales
les conditions
la proportion
Cette
dans
peut entre
culturelles
des gauchers.
peut pour
est permet-
2 0,16%. et a commence
en raison
les deux
peri-natale
symetrique
a 13,63
de
116s en
modification
sa forme
Gtre
Bcrivant
Glev&,
sigmoide
superieur
diffPre
la mortalit
2 ?i 13 sur la
d'environ
de personnes
socio-&onomique
ou en Nouvelle
bien
plus
augmentation
du pourcentage
des diffgren
exemples. intervenir augmenter
On aussi avec
le
NOTE Zusammenfassung
Personen,
die in Australien
Gesellschaftsklasse an Zahl zwischen wurde
durch
Asymptote Zeit
eine symmetrische
13,63
vielleicht
Es wird erwogen, grol3ztigigere handigkeit
Neuseeland
geboren
r 0.16
sigmoide
% gemessen Gestalt
aufgrund
fiihren kiinnen.
Kurve
wurde.
und s&&e
sozialer
ob Vertiderungen
Einstellung
sind,
einer
hohen
and mit der linken Hand schreiben,
1880 und 1969 van 2 % auf 13% zu.
hatte eine andere
Staaten,
oder
angehijren
deren
Vertiderung
obere
in der
ein als in den Vereinigten
Unterschiede
in den Vergleichsgruppen.
in den perinatalen
der Gesellschaft
nahmen
Verznderung
dargestellt,
Diese
friiher
Diese
zu einer
Risiken
Zunahme
und eine
der Links-
3932/8l/O30459~04$02.Ml/O 1981 Perpmon Press Ltd.
NOTE SECULAR
VARIATION
IN HANDEDNESS
OVER NINETY
YEARS
C. J. BRACKEYRIDGE
Department
of Psychiatry,
University
(Received
of Melbourne.
10 December
Victoria,
Australia
1980)
Abstract--Left-handed writers of high socio-economic status born in Australia or New Zealand were found to increase from about 2 to 13”,, over the period 188&1969. The change was well represented by a symmetrical sigmoid curve which allowed the upper asymptote to be evaluated as 13.63 + 0.16”,,. The observed secular variation differed in shape and began earlier than in the United States perhaps because of differences in social status between the samples. It is suggested that differential perinatal death may operate together with cultural relaxation in promoting an increasing proportion of sinistrals with time.
EFFECTof age on handedness has been measured in many samples of children. Earlier work reviewed by JONES Cl] tended to show an increase in dextrality with age, whereas more recent studies [2,3] have failed to demonstrate a sigmficant effect. Among adults increasing age was associated with an appreciable shift towards right-handedness in both psychiatric patients and healthy persons, the percentage of sinistral writers being lowest in the oldest age groups [4]. Comparable investigations on the secular variation of handedness summarized by LEVY [S] show that the frequency ofsinistrals rose monotonically in the United States by some 9”,, over the forty year period beginning from 1932 when it was 2.2”,,. This is usually attributed to relaxation of cultural pressure to be right-handed, though BROWN and JAFFF [6] have proposed the hypothesis that cebral dominance is not a state but a continuous process that develops through life. The present study reports on the increase in the proportion of sinistrality in an Australian population sample of homogeneous social status over ninety years. The purposes were to deduce the nature of the growth process involved, derive the asymptotic proportion of left-handedness for the population and compare the results with those obtained from American investigations. THE
SUBJECTS
AND METHODS
Material for the study was obtamed by distributing a questionnaire among scholars, aged from 6 to 16 yr, of seven private Melbourne Schools. Their parents were asked to provide anonymously details of the sex, date and country of birth, and natural writing hand of their children. They were requested to record the birth rank of all live births, together with still-births and miscarriages, and to exclude adopted children. In addition the same information was provided for the mother, father and four grandparents. Handedness was recorded as left, right and ambidextrous; however, because the last category could not always be distinguished from indefinite responses, only left and right-handed persons were included. Parents were financially advantaged with respect to the rest of the community and comprised a reasonably homogeneous sample of high socio-economic status. While there was considerable ethnic variation, only persons born in Austraha or New Zealand were used in the investigation. If the writing hand had been changed because of cultural pressure or injury, the original preference was requested. Justification of a binomial variable (left or right) to represent handedness has been given by ANNETT 173. To relate the proportion of left-handers to the year of their birth, the three-parameter logistic curve was used; this has the additional function ofallowing the upper asymptote to be estimated from the same observations as the slope and intercept. The procedure described by HCWLFTT and GOSTICK [8] was adopted to obtain the best fit to the data and the weighted regression method of mlnimum logit xz [9] was used for estimation and testing goodness of fit. 459
460
Differentiation
NOTE
of the logistic curve yields the equation dq dr =/Ml-Y)
1 dY dInY ;dr=~drm=p(l-i.)
or
Thus the rate of change ofthe logarithm of the proportion of left-handers varies directly with the proportion of righthanders in the population at the time (I). In the present context,y has been scaled to the upper limit of growth (YO)so that the proportionate ratio p=y/yO becomes the dependent variable in the logisticequation after division by (1 -p) and taking natural logarithms: In L 1-P the parameters YO.
requiring
estimation
being the intercept
= u+ht, a, the linear regression
coefficient band the upper asymptote
RESULTS Persons born between 1880 and 1969 were divided into seven groups and the percentage of left-handers was related to the mean year of birth as shown in Fig. 1. There is a monotonic increase from 2% in the last century to about 13.2% shortly before 1970. The symmetrical sigmoid shape of the curve implies that a logistic transformation of the dependent variable should vary in a linear manner with the year of birth. This was found to be so when a weighted least-squares analysis of the data was performed according to BERKSON[9]. which involved estimation of the three parameters: the slope (b), the intercept (a) and the upper asymptote (yO) together with a test of the goodness of fit of the transformed data to a straight line. The estimates obtained were b=0.0738~0.0021, a= - 1.236+0.064”,;, and y,,= 13.63~0.16’~. When backtransformed the best fitting curve is shown by the broken line in Fig. 1. Its acceptability is reflected in the logit 2’ value for goodness of fit of 9.00 (P> 0.1) indicating that there is no significant heterogeneity in the departures of the observed percentages from the fitted curve.
__---
o,_-= ,’
, ,
,’
p"
I’ ,’
I’
/’
,/’ ,'O /’ I’ I’
,d’ 1’ /’ /’ ,’
1890
1900
1910
1930
1920
YEAR
FIG. I.
1940
1950
1960
0
NOTE
461
DISCUSSION The symmetrical sigmoid curve describing the change in the proportion of sinistral Australians and New Zealanders over the past ninety years represents a sixfold increase which has practically levelled off by 1970. The growth process involved is that, at a given time, the rate of change of the logarithm of the proportion of left-handers in the population varies directly with the proportion of right-handers. The curve presented in Fig. 1 differs in several respects from that drawn by LEVY [S] from samples obtained in the United States. Both curves begin at about 29; but, whereas the process begins soon after 1930 in America, it has already begun by 1900 in Australasia. Both curves approach asymptotic levels by 1970, but the American data follow a parabolic, rather than a sigmoid, path. This suggests that the processes may vary in kind as well as degree in the two countries. On the other hand, the differences may reflect a disparity in social status. Thus if this is higher in the Australasian material and gives rise to a more tolerant attitude towards sinistrality, the process ofcultural relaxation should begin earlier than in a society of lower average socio-economic status. The argument depends upon a direct relationship between social class and tolerance towards sinistral writers. Such a correlation could be difficult to demonstrate if it is subject to secular variation. The present study cannot determine whether cultural relaxation is the only mechanism responsible for the large increase in left-handed writers over the period examined. Pathological left-handedness, the concept that such conditions as anoxia associated with complications of pregnancy [lo] produce an excess of left-handedness, is unlikely to be a factor. Over the past century, antenatal care and obstetric skills have greatly improved so that any effects of birth stress on handedness are likely to have operated in the opposite manner to the observed influence of secular variation. However, if birth complications associated with left handedness were regarded as potentially fatal, so that perinatal death took a greater toll of sinistral than dextral infants, the situation would be quite different. Differential wastage at the time of birth might then be a plausible mechanism acting in addition to sociocultural conditioning. Given the improved obstetric techniques and the more enlightened attitude towards left-handed writers in recent years, increased survival of sinistrals and cultural relaxation may be significant contributors to the observed secular trends.
REFERENCES 1. Jo~r:s. H. E. Dextrality as a function of age. J. esp. Ps~chol. 14, 125 143, 1931. 2. APGNETT, M. The growth of manual preference and speed. Br. J. Psycho/. 61, 545-558, 1970. 3. HARI>YCK, C., GOLIIMAU, R. and PETRINOVICFI,L. Handedness and sex, race and age. Hum. Biol. 47, 369- 375. 1975. 4. FI.I‘MIN(;FR,J. J., DALTON. R. and STANI>ACX,K. F. Age as a factor in the handedness of adults. Neurop.~~ch&girr 15,471-473, 1977. 5. LFVY, J. Psychobiological implications of bilateral asymmetry. In Hrmisphrrr Function in the Humun Brujn, S. DIMONI)and J. G. BEAUMONT(Editors). Elek, London, 1974. 6. BKOWN, J. W. and JAFFF,,J. Hypothesis on cerebral dominance. Nruropsycholoyia 13, 107 110, 1975. 7. ANNI:TT, M. Genetic and nongenetic influences on handedness. Behar. Gmet. 8, 227 249, 1978. 8. HFWLETT, P. S. and GOSTICK, K. G. The loss of weight of pyrethrin-treated flour beetles Triholiunl cusfunrum (Herbst) and its application to bioassay. Ann. appl. Biol. 43, 213--236, 1955. 9. BFRKSON, J. A statistically precise and relatively simple method of estimating the bioassay with quanta1 response, based on the logistic function. J. Am. Srarisr. As. 48, 565 599, 1953. 10. BAKAN, P., DIHH, G. and RE~I), P. Handedness and birth stress. Nrurop.~y~ho/oyiu 11, 363 366, 1973.
On a trouvd
une
pdriode
1880-1969,
la main
gauche,
Australie
de niveau
reprssentge
tant
d'dvaluer
La variation
tdt qu'aux
bien temps
que
que
courbe
l'asymptote observCe Etats
ces de conditions suggere
Zelande.
par une
Unis,
sociales
les conditions
la proportion
Cette
dans
peut entre
culturelles
des gauchers.
peut pour
est permet-
2 0,16%. et a commence
en raison
les deux
peri-natale
symetrique
a 13,63
de
116s en
modification
sa forme
Gtre
Bcrivant
Glev&,
sigmoide
superieur
diffPre
la mortalit
2 ?i 13 sur la
d'environ
de personnes
socio-&onomique
ou en Nouvelle
bien
plus
augmentation
du pourcentage
des diffgren
exemples. intervenir augmenter
On aussi avec
le
NOTE Zusammenfassung
Personen,
die in Australien
Gesellschaftsklasse an Zahl zwischen wurde
durch
Asymptote Zeit
eine symmetrische
13,63
vielleicht
Es wird erwogen, grol3ztigigere handigkeit
Neuseeland
geboren
r 0.16
sigmoide
% gemessen Gestalt
aufgrund
fiihren kiinnen.
Kurve
wurde.
und s&&e
sozialer
ob Vertiderungen
Einstellung
sind,
einer
hohen
and mit der linken Hand schreiben,
1880 und 1969 van 2 % auf 13% zu.
hatte eine andere
Staaten,
oder
angehijren
deren
Vertiderung
obere
in der
ein als in den Vereinigten
Unterschiede
in den Vergleichsgruppen.
in den perinatalen
der Gesellschaft
nahmen
Verznderung
dargestellt,
Diese
friiher
Diese
zu einer
Risiken
Zunahme
und eine
der Links-