Abstract
Background: It is widely assumed that individual differences in language lateralisation have a strong genetic basis, yet prior studies show low heritability (around 0.25) for the related trait of handedness, and two twin studies of structural brain asymmetry obtained similarly low estimates. This report describes heritability estimates from a twin study of language laterality and handedness phenotypes. Methods: The total sample consisted of 194 twin pairs (49% monozygotic) aged from 6 to 11 years. A language laterality index was obtained for 141 twin pairs, who completed a protocol where relative blood flow through left and right middle cerebral arteries was measured using functional transcranial Doppler ultrasound (fTCD) while the child described animation sequences. Handedness data was available from the Edinburgh Handedness Inventory (EHI) and Quantification of Hand Preference (QHP) for all 194 pairs. Heritability was assessed using conventional structural equation modeling, assuming no effect of shared environment (AE model). Results: For the two handedness measures, heritability estimates were consistent with prior research: 0.23 and 0.22 respectively for the EHI and QHP. For the language laterality index, however, the twin-cotwin correlations were very close to zero for both MZ and DZ twins, and the heritability estimate was zero. Conclusions: A single study showing negligible heritability for language laterality cannot rule out a genetic effect on language lateralisation. It is possible that the low twin-cotwin correlations were affected by noisy data: although the split-half reliability of the fTCD-based laterality index was high (0.85), we did not have information on test-retest reliability in children, which is likely to be lower. We cannot rule out the possibility that true heritability of differences in language lateralization is non-zero, but results indicate that the heritability of this trait is low at best. Stochastic variation in neurodevelopment appears to play a major role in determining cerebral lateralisation.
Highlights
Prior studies have estimated heritability of around 0.25 for the trait of handedness, with studies of structural brain asymmetry giving estimates in a similar or lower range
No association was found between handedness and the language laterality index: with cases divided into those with laterality indices above and below zero, 76% of the left-handers and 78% of the right-handers were left-lateralised for language
Once we move from handedness, little is known about the reliability of these different measures of phenotype, but it is clear that they are not interchangeable, and the relationships between them are not clearly understood. These points are amplified below when dealing with specific points raised by reviewers; the final paragraph of the Discussion has been amended to make it clear that we are not claiming that we have definitely proven a null result, but rather that very low, or even absent heritability of functional language lateralisation should at least be treated as a realistic contender, rather than dismissed as implausible
Summary
Prior studies have estimated heritability of around 0.25 for the trait of handedness, with studies of structural brain asymmetry giving estimates in a similar or lower range. This report describes heritability estimates using functional language laterality and handedness phenotypes in a twin sample previously reported by Wilson and Bishop (2018). A language laterality index was obtained for 141 twin pairs, who completed a protocol where relative blood flow through left and right middle cerebral arteries was measured using functional transcranial Doppler ultrasound (fTCD) while the child described animation sequences. Results: For the two handedness measures, heritability estimates (95% CI) were consistent with prior research: .25 (.03 - .34) and .18 (0 – .31) respectively for the EHI and QHP. For the language laterality index, the twin-cotwin correlations were close to zero for both MZ and DZ twins, and the heritability estimate was zero (0 - .15). It is possible that the low twin-cotwin correlations were affected by noisy data: the split-half reliability of the fTCD-based laterality index was high (0.85), we did not have information on test-retest reliability in children, which is likely to version 3 (revision)
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