Please choose to answer only one of the 2 following questions. Option 1: In your opinion and based on scientific, peer-reviewed published evidence, does child

BRIEF REPORT

Early Handedness in Infancy Predicts Language Ability in Toddlers

Eliza L. Nelson Florida International University

Julie M. Campbell and George F. Michel University of North Carolina at Greensboro

Researchers have long been interested in the relationship between handedness and language in devel- opment. However, traditional handedness studies using single age groups, small samples, or too few measurement time points have not capitalized on individual variability and may have masked 2 recently identified patterns in infants: those with a consistent hand-use preference and those with an inconsistent preference. In this study, we asked whether a consistent infant hand-use preference is related to later language ability. We assessed handedness in 38 children at monthly intervals from 6–14 months (infant visits) and again from 18–24 months (toddler visits). We found that consistent right-handedness during infancy was associated with advanced language skills at 24 months, as measured by the Bayley Scales of Infant and Toddler Development (Bayley–III; Bayley, 2006). Children who were not lateralized as infants but who became right-handed or left-handed as toddlers had typical language scores. Neither timing nor direction of lateralization was related to cognitive or general motor skills. This study builds on previous literature linking right-handedness and language during the first 2 years of life.

Keywords: handedness, language development, motor development, infant, toddler

Descending motor pathways have a crossed innervation such that the actions of the right hand are controlled by the left hemi- sphere and vice versa (Serrien, Ivry, & Swinnen, 2006; Volkmann, Schnitzler, Witte, & Freund, 1998). Manual actions are character- ized behaviorally by a marked preference for one hand over the other; typically the right hand is the preferred hand (e.g., Annett, 2002). The right shift observed in the distribution of handedness corresponds to the asymmetric distribution of speech and language in the left hemisphere. Thus, right-handedness and language are both lateralized left-hemispheric functions in most adults (e.g., Knecht et al., 2000, but see Kuhl & Damasio, 2013). Infants, like adults, are also predominantly right-handed (e.g., Fagard, 1998; Ferre, Babik, & Michel, 2010; Hinojosa, Sheu, & Michel, 2003; Ramsay, 1980). The nature of the relationship between handedness and the emergence of language skills in development remains a relatively understudied question, however.

A common approach to tackling this question developmentally has been to compare hand use for manipulative actions with hand

use for communicative actions, particularly pointing. As in infant reaching and object manipulation, a right hand bias has been reported for infant pointing (e.g., Esseily, Jacquet, & Fagard, 2011; Franco & Butterworth, 1996). Nevertheless, efforts to connect hand-use preferences across these two different contexts of hand use have yielded mixed results. Previous studies have generally found that hand-use preferences for manipulation and gesture are only loosely related, with the strongest links observed during major language gains (e.g., Bates, O’Connell, Vaid, Sledge, & Oakes, 1986; Jacquet, Esseily, Rider, & Fagard, 2012; Ramsay, 1984, 1985; Vauclair & Imbault, 2009). Although these studies have provided valuable “snapshots” of typical development, they have failed to capture and interpret the individual variability in hand-use preference seen in children followed longitudinally. In- deed, investigators are now calling for greater longitudinal efforts to examine differences in the rate of acquiring language (Vauclair & Cochet, 2012).

We propose that handedness development should be examined with a similar approach. Typically, fluctuations in infant hand use within or across test sessions have been interpreted as evidence that infant handedness is generally unstable or that handedness does not solidify until later childhood (e.g., Corbetta & Thelen, 1999; Gesell & Ames, 1947). Notably, however, recent work from multiple investigators has observed that a subset of children fol- lowed longitudinally show consistent hand-use preferences for manipulation and gesture, whereas other children were character- ized as having variable hand-use trajectories (e.g., Cochet, 2012; Kotwica, Ferre, & Michel, 2008; Michel, Sheu, & Brumley, 2002). Traditional studies that have examined small samples, single age groups, or too few measurement time points have masked these two types of children: those with a consistent hand-use preference

This article was published Online First July 15, 2013. Eliza L. Nelson, Department of Psychology, Florida International Uni-

versity; Julie M. Campbell and George F. Michel, Department of Psychol- ogy, University of North Carolina at Greensboro.

The research described in this report was supported by National Science Foundation Grant DLS 0718045 (George F. Michel) and National Institutes of Health/National Institute of Child Health and Development Training Grant T32-HD007376 (Eliza L. Nelson). The authors gratefully acknowl- edge the families that participated in this project.

Correspondence concerning this article should be addressed to Eliza L. Nelson, Department of Psychology, Florida International University, DM 256, 11200 S.W. 8th Street, Miami, FL 33199. E-mail: [email protected]

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Developmental Psychology © 2013 American Psychological Association 2014, Vol. 50, No. 3, 809–814 0012-1649/14/$12.00 DOI: 10.1037/a0033803

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and those without. What advantages, if any, might be associated with having a consistent hand-use preference during development?

Investigators from the Fullerton Longitudinal Study (FLS) have explored the relations between handedness consistency in the toddler and preschool years (18, 24, 30, 36, and 42 months) and later cognitive outcomes through 17 years of age (Gottfried & Bathurst, 1983; Kee, Gottfried, & Bathurst, 1991; Kee, Gottfried, Bathurst, & Brown, 1987; Wilbourn, Gottfried, & Kee, 2011). In this project, hand preference was measured by observing which hand the child chose to draw with while completing test items from either the Bayley Mental Scale or the McCarthy Scales of Chil- dren’s Abilities, depending on age. Children who used the same hand at all five assessments were considered consistent, and those with any differences in hand use across assessments were consid- ered inconsistent. A critical finding that persisted across infancy into adolescence in these data was enhanced verbal cognitive abilities in girls with a consistent hand preference (Wilbourn et al., 2011). Children in the consistent group were all right-handed, providing some evidence for a developmental link between right- handedness, language and the left hemisphere.

To the best of our knowledge, there are no equivalent studies of handedness consistency during infancy and emerging skills such as language. In the study reported here, we investigated whether the timing of lateralization for manipulative actions in infancy is related to language skills during toddlerhood. We also examined whether early handedness is related to advances in general motor skill and cognition. To do this, we measured handedness at monthly intervals from 6 to 14 months (infant visits) and again from 18 to 24 months (toddler visits). Language, motor, and cognitive skills were measured at 24 months using the third edition of the Bayley Scales of Infant and Toddler Development (Bayley– III; Bayley, 2006). We hypothesized that early hemispheric spe- cialization in motor skills, manifested as consistent infant handed- ness, is related to another specialized hemispheric function: the development of language. Specifically, we predicted that children with an early left hemispheric specialization (consistently right- handed infants) would have higher scores on the language scale of the Bayley than would children with no consistent hand preference during infancy.

How could having a consistent hand preference as an infant be related to language acquisition? Kotwica et al. (2008) reported that infants with a stable hand-use preference (95% of the sample were right-handed) were better multiple object “managers” than infants without a stable preference. Consistent infants transferred objects to the opposite hand more readily than inconsistent infants and also stored objects within reach while acquiring new objects. The importance of this distinction between groups in object play is that consistent infants experience the world differently than do their inconsistent counterparts, and this has potential implications for language development. Bruner (1973) argued that such object management skills depend upon, and therefore represent, a devel- oping ability of symbolic representation. Storing an object depends upon the infant’s ability to “represent” the location of the object for later retrieval.

Moreover, Lifter and Bloom (1989) observed that changes in how infants manipulate objects were linked to the timing of infants’ first words and growth in vocabulary size. In this longi- tudinal study, 14 monolingual English children were followed from 8 to 26 months of age. Infants’ actions prespeech largely

consisted of taking objects apart, but after the advent of their first words (M � 13.8 months), infants shifted to putting objects together, and they began pairing labels and their referents. During the vocabulary spurt (M � 19.4 months), object manipulation complexity increased again, and infants demonstrated increasing knowledge about the details of object properties. Although infants acquired language at varying rates, the dynamic relationship be- tween object skill and language skill was observed in all infants independent of age. As Iverson (2010) so elegantly summarized, “. . . changes in motor skills (i.e., achievements and advances in posture, independent locomotion and object manipulation) provide infants with a broader and more diverse set of opportunities for acting in the world” (p. 230).

Conceptualized in this way, consistent infant hand preference is a marker for advanced object manipulation skills, and this differ- ential early organization of motor ability may be related to the rate of language acquisition. By comparison, an inconsistent preference is an indicator of lower skill level, and perhaps a different pattern of hemispheric organization. The purpose of this study was to examine whether language outcome at 2 years could be accounted for, in part, by different patterns of change in the motor system during infancy (measured by handedness). We predicted that chil- dren who exhibited consistent right-handedness during infancy would have higher scores on the Bayley language scale as toddlers than would those without a consistent bias.

Method

Participants

Thirty-eight children (21 girls) participated in a longitudinal study involving 16 visits. Three additional children completed the infant portion of the project, but missed more than two toddler sessions and were not included in our analyses. Families were recruited for the project using birth records obtained from the local courthouse in Greensboro, North Carolina, a midsized metropoli- tan area in the southeastern United States. Study inclusion criteria included full-term pregnancy of at least 37 weeks gestation and delivery without complications. The sample was representative of the ethnic backgrounds found in the local community (sample � 65% Caucasian White, 15.8% African American, 13.2% multira- cial, 2.6% Hispanic, 2.6% other race). Families provided informa- tion regarding current income level and education attainment for mothers and fathers when available either by paper or electronic questionnaire. Yearly family incomes ranged from $10,000– $19,999 to $150,000 or more, with a median income of $70,000– $79,999. Mothers’ education level ranged from one or more years of college/no degree to a professional degree and fathers’ educa- tion level varied from a high school diploma or GED equivalent to a professional degree. The median education level for both moth- ers and fathers was a bachelor’s degree.1 The primary language spoken in the home was English, with the exception of one

1 Nonparametric Mann-Whitney U tests found no effect of family in- come (U � 56, p � .05), mothers’ education level (U � 62, p � .05), or fathers’ education level (U � 54, p � .05) on infant handedness status. Thus, identification of infants as right-handed or no preference was not due to a socioeconomic advantage in family income or parents’ education attainment.

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810 NELSON, CAMPBELL, AND MICHEL

participant, and this child was therefore not scored on the language scale of the Bayley.

Procedures and Materials

The University of North Carolina at Greensboro Institutional Review Board approved the following procedures, and parents gave written consent for their child to participate in this study. Data collection began when the child was 6 months old, and each assessment occurred within 7 days of the child’s monthly birthday. In total, there were 16 monthly visits occurring from 6 to 14 months (9 infant visits) and from 18 to 24 months (7 toddler visits). Parents received a $10 Target gift card for each visit to the lab.

To assess hand preference, we offered children one of two measures designed to recruit age-appropriate manual skills. The infant handedness measure assessed hand use for acquiring objects unimanually and consisted of 22 objects presented singly at the infant’s midline and 10 pairs of objects presented dually in line with the infant’s shoulders (for details on this procedure, see Michel, Ovrut, & Harkins, 1985, and Ferre et al., 2010). The toddler handedness measure assessed hand use for role- differentiated bimanual manipulation in which one hand stabilizes an object for the other hand’s manipulation (manipulating hand � preferred hand) and consisted of 29 scorable actions such as pulling a toy out from a container, taking the lid off a jar, or unzipping a pouch. These tasks were designed to require the use of both hands to successfully complete the target action (for details on this procedure, see Nelson, Campbell, & Michel, 2013).

We would like to note that measuring handedness in any sample requires targeting skillful behavior, and, as such, it would not be appropriate to utilize the same set of tasks over the first two years of life, when a child’s manual repertoire changes considerably. Infants are not capable of reliably performing the complex biman- ual tasks given to toddlers, and, likewise, toddlers are not suffi- ciently challenged simply reaching for objects. Thus the assess- ment of handedness in both infants and toddlers shared the same function of serving as an explicit measure of manual skill.

For both handedness measures, the child was seated at a table on a parent’s lap. Sessions were recorded with two Panasonic digital cameras that were linked by a Videonics mixer, providing over- head and left-facing views of the child’s actions that were com- bined into a single frame for later coding. Video coding of the handedness assessments was done offline by trained observers using the Observer XT program (Noldus Information Technology, v.10). Coders for the toddler data were blind to infant handedness status. Interrater reliability was calculated using percent agreement between coder pairs for each object presented (up to 34 coding decisions per infant per session). Coders scored seven–eight vid- eos from each month tested, or approximately 20% of the data. Reliability for the infant handedness measure was 93%, and reli- ability for the toddler handedness measure was 96%.

Following the final handedness assessment at 24 months, the 38 children for whom we had complete handedness data were admin- istered the Bayley Scales of Infant and Toddler Development (Bayley–III; Bayley, 2006) by a clinically trained observer. Thirty- four children completed all three scales (language, motor, and cognitive). One child could not be evaluated on language because English was not the primary language spoken in the home. One additional child (right-handed as an infant; see description of

handedness groups below) could not be tested due to illness during the eligible age period (i.e., chicken pox), and two children (one right-handed as an infant and one right-handed as a toddler) were unable to complete the Bayley due to behavioral problems. Anal- yses were conducted on the composite score for each scale. Scales are normalized at 100, with a standard deviation of 15.

Results

To determine handedness, we first calculated the child’s per- centage of right-hand use (%R) from each visit using the formula [R/(R � L)]�100, where R is the number of right-hand actions and L is the number of left-hand actions. Next, we computed 95% confidence intervals (CI) derived from each child’s monthly right- hand use percentages for his or her block of infant visits (Visits 1–9 from 6–14 months) and separately for their block of toddler visits (Visits 10–16 from 18–24 months). Children were classified as left-handed if their mean %R � CI � 50% and right-handed if their mean %R – CI � 50%; values that were within 5% of the 50% level were also considered lateralized. Children were classi- fied as having no statistically reliable preference if their mean %R � CI crossed the 50% level by more than 5%.

We assigned an infant handedness status and a toddler handed- ness status to each child who completed the Bayley (N � 35). By infant status, 13 children were right-handed and 22 had no pref- erence. By toddler status, 26 children were right-handed, 8 were left-handed, and 1 had no preference. Using the relationship be- tween infant handedness status and toddler handedness status, we grouped children into three handedness trajectories: (a) early right- handed (n � 12): children who were consistently right-handed as infants and stayed consistently right-handed as toddlers; (b) late right-handed (n � 14): children who had no preference as infants but became right-handed as toddlers; and (c) late left-handed (n � 8): children who exhibited consistent left-handedness as toddlers. This last group included one child who had been classified as right-handed as an infant but became left-handed as a toddler. No other child switched handedness status. There were no children in the sample with a left-hand preference as infants. Finally, one child did not exhibit a consistent hand preference by the conclusion of the study. Data from this child were used in the infant analyses but not in the toddler analyses, given the insufficient cell size for the no-preference toddler group.

We used independent samples t tests and one-way between- subjects analyses of variance (ANOVAs) to examine the effects of gender (male or female), infant handedness status (right-handed or no preference) and handedness trajectory (early right-handed, late right-handed, late left-handed) on cognitive, language, and motor abilities at 2 years of age, as measured by the Bayley. There was no effect of gender on cognitive level, t(33) � �0.284, p � .05, d � 0.10, language level, t(32) � �0.252, p � .05, d � 0.09, or motor level, t(33) � �0.169, p � .05, d � 0.06. Means, standard deviations, and ranges for each Bayley scale grouped by infant handedness status and by infant to toddler trajectory status are given in Table 1. We found a large effect of infant handedness status on language outcome at 2 years. Children who exhibited consistent right handedness as infants scored significantly higher on the language scale compared with their later lateralized coun- terparts (children with inconsistent hand use as infants), t(31) � �2.187, p � .05, d � 0.77. However, early right-handed infants

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