NOTE

NO LINK BETWEEN LEFT-HANDEDNESS AND MATERNAL AGE AND NO

ELEVATED ACCIDENT RATE IN LEFT-HANDERS

MICHAEL PEEKS* and RHONUA PEKKY

Department of Psychology, University of Guelph. Guelph, Ontario, Canada NIG 2Wl

(Receired 30 May 1991: accepted 9 Septrmhrr 1991)

Abstract-No significant link was found between maternal age and handedness of offspring for 154 left-handers and 560 right-handers. Questionnaire data on traffic accidents for 302 left-handers and 719 right-handers failed to support the claim that left-handers have an elevated risk for accidents.

COKE% and HALPEKS [3] make a general case that left-handedness is a marker for reduced fitness. Given that careful reviews of the question of the pathological nature ofleft-handedness (e.g. Ref. 141) question whether pathology plays a role in the majority of left-handers, the statements by Coren and Halpern come as somewhat of a surprise. Because of the practical importance ofCoren and Halpern’s claims, the data on which they are based merit close examination and, if possible, corroboration in independent studies. We have a data set that allows an independent evaluation of two of Coren and Halpern’s assertions. The first assertion holds that the relative risk of left-handedness rises with maternal age ]I]. MCMAN~JS [15] has already expressed some doubts about the reliability of this particular claim. The second assertion holds that left-handers have a higher risk for accidents than right-handers [2]. We have collected data sets for 154 left-handed and 560 right-handed university students that include information on handedness, and maternal age. We also have questionnaire data on accidents and handedness from 302 left-handers and 719 right-handers.

Because we are interested in left-handers. there was a higher proportion of left-banders in the sample than normally expected, but this does not introduce a systematic bias. An unexpected difficulty in relating our data to Cot-en’s work was that there are no indications in COKEN’S [l] study as to how he classified handedness. Coren mentions that hc had available to him “drawing. throwing, dealing out cards and using eraser” as handedness preference items but he does not specify how he used this information to classify handedness. An additional ditliculty is that Coren does not give the number of left-handers in his calculations so that it is not even possible to infer his classification scheme.

METHOD

Data were collected from 714 university students, with information ofhandedness, maternal age and accidents. Of these subjects. 217 were male and 497 were female; 154 were left-handed and 560 were right-handed. There were additional data containing information only on handedness and accidents from 143 left-handers and 112 right- handers. Of these. 152 were female and 103 were malt.

All subjects had completed a 12-item questionnaire that asked for hand preferences for the following activities: write, hammer, throw. unscrew lid ofjar, use knife for cutting bread, use toothbrush, hold match while striking, hold racquet, USC scissors, draw, USC spoon, hold needle when sewing on button. Over 600 of the subjects had also completed a 60-item handedness questionnaire which also included the item used by Coren ‘which hand holds

*To whom correspondence should be addressed

1257

125X NOII

crascr’. Information was also glvcn on the subject‘s age and on the age of the subject’s mother. In addition. subjects answered the question ‘Were you involved ma traRic accident in the last two years (ixcyclc. motor hike. car), whcrc you wcrc the driver’? This refers to any accident where thcrc was damage to it person or vchiclc’.

An cstimatc of the reliability ofsubject’s judgment of maternal age was made by follow-up home phone calls to 40 randomly chosen subjects. Of these, 9 had made errors in the initial statement of their mother’s age. Howcvcr. the average error was less than 2 years and we judged the information sufiiciently rcliablc to he used for analy~ia.

RESULTS How to define left-handedness in the absence of any procedures described by Coren? We decided to define ah

left-handed those who write with the left hand. Our reasoning was as follows. The item ‘draw,’ was perfect]) corrclatcd with ‘write’ for Icft-handcrs in our sample (two of the 560 right-handed writers drew with the left), so that WC could ube ‘write’ instcad of ‘draw’. The choice of the ‘throw’ item by Coren IS unfortunate because a significant number ofleft-handers throw with the right arm [7]. IfCoren had classified as left-banders all those who performed all four of his items with the left hand he would have obtained a smaller proportion of left-banders [6] than if he had classified them according to writing hand only. As examination ofjust this group gave results no different from the ones we will discuss. we decided on ‘writing hand left’ as the sole criterion for left-handedness. Using this criterion. WC found that the average age of the mothers of left-banders was 27.2 years whereas the average birth age of mothers of right-handcrs was 27.5 yearh. The corresponding ages given by COKEN [I] were 27.9 and 27.4 and those given b> McMnnus [S] wcrc 27.2 and 27.5. Thus, at least by this criterion. the three samples were surprismgly well matched.

The data given in Table 1 show why maternal age is not a good measure for distinguishing between right- and left- banders. Only 2.3X”/o ofall offspring are horn of mothers 40 years of age and older. Thus, even if there were an cxccs!, of left-handcrs. this would not express itself in a dctcctable group age difference for mothers because there would bc very few mothers of Icft-handcrs in this group. We further note the source for a potcntlal sampling artifact. If only 2.3X’% of all offspring are born of mothers 40 and older. and assuming a left-handedness prcvalcnce of IO%, one would expect tivc Icft-banders born of these mothers in a sample of 2000 subjects. Ohviouslq, an additional Icft- bander more or less would appreciably alter the relative risk calculations. Coren’s claim that mothers in this age group arc more than twice as likely to give birth to left-handers than younger mothers is less impressive If looked at 111 this way.

In Table 1 wt: prcscnt the numbers ofolTspring born ofmothers in the five age catcgorica used by Coren. together- with the numbers ofleft-handcrs and right-banders in each age group. In addition, we have indicated the rclatibc risk of being born as left-handcr in the age groups, together with the ligures given by Corcn [I]_

Table I. Proportions of left-banders and right-banders born of mothers in five age groups.

with relative risk (RR) of being left-handed in each group

Maternal age Left-handed Right-handed % of births RR

17 24 43 161 28.58 I .oo (I .oo)* 25 29 67 231 41.74 I .06 ( I. I I ) 30 34 33 114 70.59 I .07 (I 25) 35 39 X 30 6.72 0.80 (I .69) = <40 3 I? 2.3X 0.84 (2.28)

Total 154 560

*F‘igurcs 111 brackets are <‘OK1 I’S 111 relative risk coeflicients.

There ~b no indication in our data that older mothers have diaproportionatcly more left-handed oltbpring. A non-significant &-square value of I .03 indicates that right- and left-banders are distributed in equal proportions across the maternal age groups. It should be pointed out that the relative risk quotient gives quite misleading impressions when IOU cell numbers arc concerned. For lnstancc. the relative risk of0.82 for left-handed offspring to bc born of mothers 40 years of age or older looks as if this is qultc a departure from a ratio of I .OO. However. in performing a &-square. expected values of 3.66 left-handed offspring and 13.3 right-handed offspring of these mothers are computed. which is bery close to the actually observed three left-banders and 14 right-handers in these CCIIS.

Table 2 indicates the numbers of subject\ who report accidents. Separate tests wcrc performed for males and females. There was no significant dilKerence in incidence of accident5 between left-handed males and right-handed mules (Y’ ~0.66. ns). Thcrc was a significant differcncc between left-handed fcmalcs and right-handed females (Y’ -4.66. 1’<0.05). As Table 2 indxates. this was due to the fact that female Icft-banders show a relatively smaller proportion of individuals who reported accidents than female right-handers.

NOTE 1259

Table 2. Incidence of self-reported traffic accidents in right- and left-handers reported over the span of 2 years

Traffic accidents involving material or personal damage Yes No

Handedness Left Right Left Right Sex F M F M F M F M N 29 24 95 13 166 83 348 203 % 14.9* 22.4 21.4 26.4 85.1* 77.6 18.6 13.6

*Refers to percentage of individuals within each sex/handedness group that did or did not report a traffic incident.

CONCLUSION Our sampling methods are not adequate by actuarial or epidemiological standards and we would not have

published these data under normal circumstances. However, the nature of the data used by Coren to claim differences between right- and left-handers does not substantially differ from the data collected in our study, and Coren’s claims have important implications in terms of ‘the real world. The data fail to reveal a significant association between maternal age and handedness of offspring. With regard to the traffic-accident data, female left- handers show a lower incidence of accidents than female right-handers. It is advisable not to overinterpret this particular result unless it is replicated with larger samples and better epidemiological procedures. In part, our reluctance to speculate is due to the fact that the question pertaining to traffic accidents was phrased in a very general way, allowing for very subjective interpretations of what ‘material or personal damage’ means.

No claim is made here that differences between right- and left-handers do not exist. We merely assert that there is insufficient evidence to support the claims made by Coren. Finally, use of the term ‘risk’ with regard to the prevalence of left-handed offspring in the different maternal age groups merely reflects epidemiological usage for the purpose of calculations and does not imply that left-handedness in itself is a risk factor.

Acknorvlrdyement~This work was supported by National Sciences and Engineering Research Council of Canada Grant No. A 7054 to M. Peters.

REFERENCES 1. COREY, S. Left-handedness in offspring as a function of maternal age at parturition. Thu New England Journal o/

Medicine, 322, 1673, 1990. 2. COREY, S. Left-handedness and accident-related injury risk. Am. J. pub/. Health, 79, l-2, 1989. 3. COREY, S. and HALPEKN, D. F. Left-handedness: A marker for decreased survival fitness. Psychol. Bull. 109,

9&106, 1991. 4. HAKKIS, L. J. and CARLSON, D. F. Pathological left-handedness: An analysis of theories and evidence. In Brain

Lateralization in Children, D. L. MOLFESE and S. J. SEGALOWITZ (Editors), pp. 289 372. Guildford Press, New York, 1988.

5. MCMANUS, 1. C. Left-handedness and maternal age. New EnqI. J. Med. 323, 14261427, 1990. 6. PETERS, M. (1900). Phenotype in normal Left-handers: An understanding of phenotype is the basis for

understanding mechanism and inheritance of handedness. In Ltft-handedness: Behavioral implications und anomalies, Adoances in Psychology Series, S. COKEN (Editor), pp. 167-192, Elsevier, Amsterdam, 1990.

7. PETEKS, M. (1990). Subclassification of left-handers poses problems for theories of handedness. Neuropsycholo- gia, 28, 279 289, 1990.