1. Introduction
Infants spend around 60% of their daily routine with objects [1,2]; these interactions are essential for learning [3]. How infants engage with objects changes dramatically across the first year of life [4]. Starting at around 4 months of age, infants can successfully reach and grasp objects [5], and this behavior (termed acquisition), is used to index handedness, starting at 6 months [6]. Not all infants have a statistically identifiable hand preference during early development; rather, hand use is variable in a subset of infants [7,8,9]. This difference in patterning for how the hands are used may be developmentally meaningful. The goal of this study was to test whether lateralization, defined as a left- or right-hand preference, confers an advantage for early object skill. We explored whether infants with a hand preference, regardless of direction, showed different rates of growth in how they interact with objects compared to those without a hand preference, using a longitudinal design that measured hand preference and object skill over seven visits from 6 to 12 months.
1.1. Handedness in Infants
Handedness in infants does not look like handedness in adults. The prevalence of left-handedness in adults is ~10%, regardless of whether handedness is measured as two categories (right–left) or three categories (right–mixed–left) [10]. What is distinctive about adult handedness is the marked preference for the right hand (80–90%). Furthermore, adult handedness is typically measured once, and often with a self-report questionnaire, for example [11]. By comparison, infant handedness varies [12,13], and how it is measured also varies. Handling this variability appropriately requires a longitudinal study design to capture the different trajectories, or patterns, of change over time in infants’ manual abilities that are also changing over time [14]. Large-scale studies using latent class analysis to parse the variability in infant hand use into patterns have estimated that 38–57% of infants are right-handed, 5–14% are left-handed, and 30–50% have no identifiable hand preference [7,8,9].
The difference in how hand preferences are distributed across infants versus adults suggests that handedness is not innate but, rather, develops. This process is hypothesized to be the result of cascading events across prenatal and postnatal development [15,16,17,18,19]. Briefly, there is a correspondence between the fetal positioning bias, measured at birth, and the postnatal supine head-turn preference [20]. Head-turn preference subsequently predicts the hand preference for acquiring objects [21,22,23]. Acquisition hand preference predicts the hand preference for manipulating objects with one hand [24,25] and, finally, unimanual hand preference predicts the hand preference for manipulating objects with two hands [26]. Each asymmetrical experience cascades into the next increasingly complex motor skill across development. But why have a hand preference at all as an infant? The current study explores whether there is a benefit to lateralization in infancy for handling objects.
1.2. Lateralization and Object Skills in Infants
A small body of literature has examined the effect of lateralization, measured as infant hand preference, on the performance of object skills such as stacking, bimanual manipulation (termed role-differentiated bimanual manipulation or RDBM), and tool use. Marcinowski and colleagues [27] reported that infants with a hand preference were more successful in early stacking, as measured by the age of stacking skill attainment and the height of the block tower, compared to infants without a hand preference. In a follow-up study measuring the same sample again as toddlers, those children with a consistent hand preference from infancy to toddlerhood were more successful at stacking than children with a variable or inconsistent hand preference across the two developmental time periods [28]. Together, an early and consistent hand preference is an advantage for the development of stacking objects.
Mixed findings have been reported in the research on infant hand preference and RDBM, where one hand holds the object for the other hand’s manipulation. In a study examining RDBM speed, Campbell and Marcinowski found that infants with a right hand preference improved their RDBM performance at a faster rate relative to the left-handed or no preference infants [29]. However, there was no similar advantage for left-handed infants who did not differ in their RDBM speed from the no preference group. Babik and colleagues reported a negative association between hand preference strength and RDBM in infants, suggesting laterality may disadvantage the performance of a skill where the hands must be coordinated together, as opposed to stacking, which is often executed with one hand [30]. Moreover, this study did not find a link between hand preference strength and tool use, leading the authors to suggest that whether lateralization affects infants’ object skills depends on a task’s demands, such as the level of dexterity required and whether one or both hands is used.
An additional task demand to consider when examining the potential influence of laterality is whether there is more than one object (termed object management). Acting on more than one object is likely a combination of two separate skills: acquisition and storage (e.g., transferring an object from one hand to the other or placing an object within reach without dropping it) [31]. All infants demonstrate an increase in their ability to manage multiple objects from 7 to 13 months, but infants with a stable hand preference were found to do so at a faster rate, with more sophisticated object management sequences, than infants without a hand preference [32]. In this prior study by Kotwica and colleagues, the infants were sampled every other month and hand preference groups were descriptive, rather than statistically-driven, due to the small sample size (n = 38). The current study addressed these gaps by sampling infants every month across a similar age range (6–12 months) and with a larger sample (n = 303) that permitted group-based trajectory modeling to determine hand preference groups. Object management skill was assessed using the goal-directed motility of arms and hands item from the Touwen Infant Neurological Examination (TINE) [33], which allowed us to track the inter-individual variabilities in the infants’ ability to hold one or more objects.
1.3. The Current Study
The current study was motivated by the following main research question: Do infants with a hand preference have an advantage in the development of early object skills? We tested our hypothesis that lateralization confers an advantage in object skill growth. We predicted that the infants who were lateralized would acquire multiple objects sooner, a marker of more complex object management skills, than the infants with no hand preference. The basis for this prediction was the expected differences between the hands for infants with a hand preference versus those without a hand preference. A hand preference creates an asymmetric hand experience. The hand that is used preferentially becomes more practiced, and thus more proficient, than the non-preferred hand, and more adept at interacting with its environment. Thus, an infant with a hand preference can manipulate objects in their environment more readily with their preferred hand [34]. In contrast, the non-preferred hand is less-experienced and not as “good” as the preferred hand because it is not as practiced as the preferred hand. An infant without a hand preference is likely to have poor motor precision in both hands, because neither hand is preferentially used [35]. This proposed difference in how infants acquire one object may have consequences on how infants interact with multiple objects.
In the current study, we measured hand preference and object management skill (e.g., holding zero, one, two, or three objects) using a longitudinal design, with monthly assessments from 6 to 12 months of age. We expected that a subset of infants would be lateralized and exhibit a left- or right-hand preference trajectory across the seven visits. We further predicted that the lateralized infants would be able to hold and manage objects more effectively (i.e., achieve “higher” object management scores), showing a different pattern of growth across visits by acquiring multiple objects sooner, and developing the ability to hold more objects more quickly compared to infants without a hand preference who were not lateralized.
2. Methods
2.1. Participants and Procedure
The infants (n = 303) were recruited using Guilford County public birth records (North Carolina, USA). In total, 169 infants (55.78%) were male, and 134 were female (44.22%). All were from full-term pregnancies and births without complications. Infants were brought to the Infant Development Center at the University of North Carolina, Greensboro within seven days of their birth date from 6–12 months at monthly intervals as a part of a larger longitudinal project consisting of over 480 infants. The sample used in the current study was an earlier dataset of infants with no more than one missing visit at the time of analysis. The procedures for recruitment, obtaining informed and written consent, and data collection were in accordance with the regulations set by the UNCG Institutional Review Board for the protection of human subjects (project title: Development of Infant Handedness; IRB #05-0071). On the first visit, parents gave their written consent for their child to participate in the project. All testing took place in a lab in a quiet environment. After a warm-up period, the trained experimenters first administered the TINE and then the hand preference assessment. The parents received a $10 Target gift card at each visit.
2.2. Measures
2.2.1. The Touwen Infant Neurological Examination (TINE) [33]
At each visit, infants were administered 11 test items from Group III of the Touwen Infant Neurological Examination (TINE) [33]. The TINE has a good predictive validity and reliability [36,37]. The TINE was given by members of our research team who were trained to more than 90% intra-rater and inter-rater agreement on 24 test infants of various ages, ensuring confidence in our ability to detect developmental changes with this measure.
For the current study, we used one item, goal-directed motility of arms and hands, for hypothesis testing. For a similar approach, see [38,39]. This TINE item measures the infants’ object management ability by assessing their ability to hold up to three objects. The TINE was given on a mat on the floor. If the infant could not sit independently, they were supported by their parent or the researcher. A researcher would encourage the infant to hold toy eggs measuring ~14.5 cm wide by 15.5 cm tall. First, the researcher would tap two eggs together to make a noise and offer them to the infant on a flat palm. If the infant did not show any interest in the eggs, the researcher would push an egg into the infant’s hand. Once the infant had warmed up to play, the researcher would offer the toy eggs again in the same manner as above. If the infant was able to hold one egg, a second egg was offered. If the infant was able to hold two eggs, a third egg was offered.
The researcher scored the most advanced holding behavior the infant demonstrated within 5 min on the mat. Higher scores indicated a more advanced object management skill. Scoring was conducted in real-time, according to the testing manual. Which hand(s) the infant used for holding was not recorded, because this information is not collected during TINE administration. The infants received a single score per assessment, ranging from 0 to 6, where 0 was “no goal-directed motility of arms and hands”, 1 was “looked at and playing with the hands but did not engage with the first object”, 2 was “touched object presented but did not hold it”, 3 was “played with feet”, 4 was “holding one object”, 5 was “holding an object in each hand”, and 6 was “holding two objects in one hand such that the infant was able to grasp a third object without dropping the others”. We dropped “3” from the original TINE scale because it did not capture the infants’ object interactions and no infants in our sample were given this rating. We regrouped the remaining scores into four holding categories for analysis (Table 1).
2.2.2. The Hand Preference Assessment [6,9] and Classification
At each visit, hand preference was assessed using a reliable and validated procedure developed by Michel and colleagues [6,9]. The experimenter sat directly across from the infant on the convex side of a rounded crescent-shaped table, while the infant sat on the concave side, seated on their parent’s lap. The infant’s navel was at table height, leaving their arms unconstrained. The parent sat close to the table and held the infant on either side of the infant’s waist to maintain a stable posture. Video cameras (Panasonic WV-CP240) were placed to the left side and directly above the infant’s hands, allowing for two views synchronized into a single frame for coding. If the infant became fussy, a break was taken. If the visit could not be completed due to fussiness, another appointment was scheduled within 5 days, and the measure was restarted at the second visit.
Thirty-two objects of varying shapes and sizes were presented to the infants one-by-one. These objects were meant to interest the infant enough to entice them to interact with the objects. The objects were presented either in singles (26 objects) or in pairs (6 objects). Single objects were presented either on the table at the infant’s midline (29 objects), or in the air (3 objects), in line with the infant’s nose. The in-air presentations were held 20 cm from the infant’s shoulders and 12–15 cm above the table. The paired objects were two identical objects placed on the table in line with the infant’s shoulders. The presenter allowed the infant to manipulate each object until its acquisition (i.e., successful reach and grasp) or until 20 s had passed (whichever occurred first). The entire task lasted approximately 10–15 min.
The hand used to acquire the object(s) in each presentation was coded from the videotapes using Noldus © Observer XT 10.1, which allows coders to stop or slow down the videos for coding accuracy. In 20% of the randomly-selected videos, the overall reliability was 93.22% agreement for inter-rater and 97.9% agreement for intra-rater reliability. For the analyses, hand use was used to calculate a proportion for each infant at each visit,
(1)
where-
R is the number of right-hand contacts,
-
and L is the number of left-hand contacts.
Next, the hand use preference patterns for each infant were classified through Group-Based Trajectory Modeling (GBTM) using the SAS TRAJ procedure [40]. For a similar approach, see [40,41]. GBTM is a statistical technique that clusters similar trajectory patterns together and identifies sub-groups whose members follow a similar developmental trend [42]. Given that this technique assumes that observations are drawn from a population with distinct sub-groups, the sub-groups may be qualitatively different within a population, but relatively homogeneous within the sub-group [43].
2.3. The Analytic Plan
We used a type of Hierarchical Generalized Linear Model (HGLM) called a multilevel ordinal longitudinal model. This type of model is used with ordinal data and other non-normal outcomes. Data using an ordinal scale gives arbitrary numerical values, which only serve to differentiate a nonspecific increase from one level to the next [44]. A HGLM converts the outcome variable into a logit link function, which makes it predictable using regression. A model using an ordinal outcome also assumes that differences in odds from one step to the next are the same across all levels of all covariates. The model-building strategies recommended by Raudenbush et al. [45] and Singer and Willett [46] were employed in the development of the models. We started with the conditional growth models with all predictors, followed by model reduction. For all analyses, a conventional α-level of 0.05 was the criterion for gauging statistical significance.
The independent variables were Age, Age2, and Handedness. For inclusion into our model, Age was centered at 6 months (0, 1, 2, 3, 4, 5, 6). Age2 was calculated by squaring centered Age (0, 1, 4, 9, 16, 25, 36). The Left and Right preference groups were dummy-coded with no preference (NP) infants serving as the reference group to test our main research question about laterality influencing change in holding growth patterns over time. In other words, we compared Left to NP and Right to NP, but did not compare Left to Right (i.e., two lateralized groups). The dependent variable was Holding. Holding had 4 ordinal categories: 0 (no holding), 1 (holds 1 object), 2 (holds 2 objects), or 3 (holds 3 objects). The variance components were tested at the intercept, linear age, and quadratic (age2) slopes to assess changes in individual variability.
3. Results
Three unique hand preference trajectories were identified in the GBTM according to BIC values, cf. [47]: left, right, and no hand preference (Figure 1). Just over half of the sample (54.1%) was lateralized, meaning that we identified a stable left- or right-hand preference in those infants. Of these, 36 infants (11.9%) were left-handed and 128 infants (42.2%) were right-handed. The remaining 139 infants (45.9%) were classified as having no stable hand preference from 6 to 12 months of age.
Left-handers (M6m = 0.47, t(35) = −0.87, p = 0.27) exhibited equal hand use (0.5) at 6 months; however, left-handers had a decrease in their right hand use over time (r1 = −0.02, p < 0.01). Right-handers (M6m = 0.67, t(124) = 9.97, p < 0.01) had a right-hand preference from the first assessment. Right-handers exhibited a positive linear (r1 = 0.06, p = 0.03) and quadratic change (r2 = −0.01, p = 0.03). Infants with no preference had significant left hand use at the start of the assessment period (M6m = 0.46, t(131) = −2.41, p = 0.02), but had an increase in their right hand use linearly (r1 = 0.01, p < 0.01). Infants with no hand preference first exhibited a right bias at 10 months (M10m = 0.54, t(138) = 2.26, p = 0.03).
As expected, the infants transitioned from no holding to holding objects from the age of 6 to 12 months. No holding (29.3% of the infants) occurred most frequently at the six months visit across the entire sample and declined to 2.0% of the infants at the nine months visit. The percentage of the infants only capable of holding one object declined from 47.6% at 6 months to 5.6% at 12 months. Holding two objects transitioned from 23.1% of the infants at 6 months to its highest value of 89.7% of the infants at 10 months. Holding two objects stayed at this plateau, with 86.6% of the infants holding two objects at 12 months. The incidence of holding three objects increased only mildly from 9 to 12 months (0.6–7.4%).
Age (β10 = −1.02, p < 0.01) and Age2 (β20 = 0.03, p < 0.01) were significant predictors of holding, indicating that the log-odds for exhibiting different behaviors of holding changed quadratically over the 6–12 month period. The variance components for Age (u1 = 0.14, p > 0.50) and Age2 (u2 = 0.01, p > 0.50) were not significant. The variance component for the intercept was significant (u0 = 0.82, p < 0.01). Finally, the reliability, or our confidence in our estimate of the intercept for holding was 0.483.
Left-handers differed in their linear (β11 = −0.82, p < 0.01) and quadratic (β21 = 0.17, p < 0.01) slopes from the infants without a hand preference (Table 2, Figure 2). Right-handers also differed in both their linear (β12 = −0.54, p < 0.01) and quadratic (β22 = 0.11, p < 0.01) slopes from the infants without a hand preference. Neither group differed at the intercept (ps > 0.32). The transition from holding one to holding two objects occurred more quickly for infants with a right- or left-hand preference than for infants with no hand use preference (Figure 2).
4. Discussion
We predicted that lateralized infants (i.e., those with a left- or right-hand preference) would have an advantage in the development of object skills indexed as the ability to hold one or more objects. We measured hand preference monthly from an established infant measure and found the following three trajectory patterns using GTBM: left, right, and no preference. Using HGLM, we then compared the intercepts, linear slopes, and quadratic slopes across the seven visits for object skill ratings between the lateralized and non-lateralized infants. Our prediction that lateralized infants would have an advantage in object handling was partially supported. While the infant hand preference trajectory groups did not differ from each other in where they started on object skill ratings, we found differences between the lateralized and non-lateralized infants in the rates of change in holding one versus two objects over time. These findings expand our knowledge of lateralization and motor cascades in infancy.
Our prediction that there would be variability in the hand preference trajectories and that only a subset of the infants would be lateralized was supported. Just over one-half of the infants in our sample were lateralized. Among the lateralized infants, the majority preferred their right hand, with a minority preferring their left hand, mirroring the trend observed in adults. This finding aligns with prior studies reporting multiple patterns in the development of infant handedness. Using a sophisticated statistical approach appropriate for our large sample size, we parsed this variability into three hand preference groups, and our estimates of the percentage of infants in each trajectory fell within the ranges computed from prior studies [7,8,9]: left (12% in the current study; range 5–14%), right (42% in the current study; range 38–57%), and no preference (46% in the current study; range 30–50%). Taken together, we are confident that we measured infant handedness rigorously and characterized hand preference accurately. However, there could still be influences from biological factors such as those related to genetics or hemispheric specialization. Although genome-wide association studies have identified genetic variants associated with handedness in adults [48], no such studies exist for infants. A recent large-scale analysis of structural brain asymmetries in children found differences between the hemispheres [49], but their sample did not contain children younger than 1 year to compare them with the age of the children in our project.
With regards to object skill, the complexity with which infants handled objects increased from 6 to 12 months of age, which was consistent with Touwen’s original observation [33]. At 6 months (i.e., intercept), there was some variability in the infants’ ability to hold objects, but this variability was not associated with the infants’ classification into a lateralized or non-lateralized hand preference trajectory. It could be that at 6 months, lateralized infants have not yet accumulated enough experience with their preferred hand. However, future work is needed to precisely measure the “experience” of each hand during interactions with everyday objects in the infants of different hand preference trajectories to test this possibility. Unfortunately, we do not have data on which hand(s) the infants used for holding during the TINE to examine if there were differences in hand experience between the groups. It is possible that one benefit of lateralization would be that infants gain experience, or skill, in holding more quickly when using their preferred hand as opposed to switching.
The infants accelerated their rate of change in holding at an early age, from 6–9 months, with the rate of change decreasing after 10 months. The quadratic slopes were relatively similar across individual infants. Infants who did not do any holding virtually disappeared by 7 months. The incidences of holding one object also decreased between 7 and 10 months until only a small number of the infants received this score as their highest rating at 12 months. A complimentary pattern was observed for handling two objects. The incidences of holding two objects increased rapidly between 6 and 10 months before leveling off. The timing of this plateau is consistent with the cognitive and movement/physical developmental milestones set by the Center for Disease Control and Prevention (CDC) in the United States [50]. According to the CDC’s “Learn the Signs. Act Early.” campaign, a milestone indicates that 75% or more of children are expected to be able to perform the skill. At 9 months, children should demonstrate several of the milestones involved in managing objects, such as “bangs 2 things together” [51,52,53], “looks for object when dropped out of sight (like his spoon or toy)” [51,54], and “move things from 1 hand to her other hand” [52,53,54,55,56,57,58,59,60,61]. The odds of infants holding three objects were relatively low even by 12 months, suggesting that future research should continue to track the growth in the management of multiple objects beyond 1 year of age to capture further increases in object skills. It is possible the reason the odds of holding three objects were low in this study was because it was difficult for infants to hold two of the TINE objects in one hand. While we acknowledge that hand size is important to consider when evaluating holding, we do not think hand size is a significant factor for the ages of the infants in this study, and thus, we did not record hand size in this project. Research studying holding patterns extending beyond 1 year of age should take hand size into account.
While all the infants showed similar developmental trends in their object management skills over time, the lateralized infants had an advantage. The Left and Right infants differed in both their linear and quadratic slopes from the NP infants. These differences in the rate of change mean that the lateralized infants transitioned from holding one object to holding two objects more quickly than the non-lateralized infants, and suggest that handedness plays a role in the infants developing object handling skills over this period. This interpretation is consistent with a prior study by Kotwica and colleagues [32], where the infants with a stable hand preference, similar to the lateralized groups in the current study, had more object storage skills and developed storage skills at an accelerated rate relative to the infants without a stable hand preference. These storage skills, transferring an object from one hand to the other hand and placing an object within reach for later use, are critical components for successfully managing multiple objects and provide the foundation for the development of RDBM [62]. Taylor and colleagues [39] recently found that infants’ object skill at 6 months predicted growth in RDBM from 9 to 14 months, although these authors did not examine laterality. Whether lateralized infants have a cascading and continued advantage in RDBM growth remains unknown. Prior studies have only examined RDBM performance (measured as speed [29] or type of RDBM performed [30]) and lateralization. Exploring this question of whether infant lateralization influences RDBM growth would be noteworthy during the period between 11 and 18 months, when a hand preference for RDBM is thought to develop [63].
5. Limitations and Future Directions
One limitation of our approach is that we did not test for differences between hand preference groups in each month in our repeated-measures design. We reported whole-trajectory analyses because within-visit testing is not very powerful with ordinal data. Moreover, our a priori predictions were focused predominantly on changes over time. We did not have a reason to examine any specific visit on its own, beyond the first assessment to determine the infants’ starting points for object skill and acquisition hand preference. A second limitation is that we did not film the TINE assessment. Thus, we were unable to examine the differences between the groups for object-handling strategies or sequences like in [32].
We encourage other investigators interested in questions regarding infants’ object experience to pursue longitudinal, rather than cross-sectional or single-timepoint designs, to further characterize the developmental cascades in infancy. In this theoretical framework, development is defined by four major tenets as continuous, interconnected, cumulative, and context-dependent [64]. A change in one domain may have implications both within and across multiple domains and on multiple timescales [65]. Although we described a motor cascade in the current study, it is possible that the lateralized infants have advantages in other domains. Indeed, an emerging area of interest in developmental science is motor-language cascades [66]. Several studies have connected hand preference trajectories to later language outcomes [67,68,69]. Future research might consider connecting object experience, laterality, and language skills across childhood.
6. Conclusions
Drawing on a rich longitudinal study design with a large sample size, we have shown that lateralized infants have an advantage in early object skill development compared to non-lateralized infants. Whether this advantage persists downstream to more complex object handling like RDBM or across non-motor domains merits further investigation.
Conceptualization, E.C.M. and G.F.M.; formal analysis, E.C.M.; data curation, E.C.M.; writing—original draft preparation, E.C.M. and E.L.N.; writing—review and editing, E.C.M., G.F.M., and E.L.N.; supervision, G.F.M.; project administration, G.F.M.; funding acquisition, G.F.M. All authors have read and agreed to the published version of the manuscript.
The study was conducted in accordance with the Declaration of Helsinki, and approved by the Institutional Review Board of the University of North Carolina Greensboro (protocol #05-0071).
Parents gave informed consent for their child to participate in this study.
The raw data supporting the conclusions of this article will be made available by the authors upon request.
We are grateful to all the participating families. We also thank past members of the UNCG Infant Development Center for their assistance with collecting or coding data.
The authors declare no conflicts of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of the data, in the writing of the manuscript, or in the decision to publish the results.
Footnotes
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Enlarge this image.Figure 2. The probability of holding across the ages by hand preference group. The No Preference (NP) group is represented by dotted lines. The Left group is represented by solid lines. The Right group is represented by dashed lines.
Recategorization of object management skill into holding categories for statistical analyses.
| Holding Category | TINE Score and Description of Rating |
|---|---|
| No holding (0) | 0—No goal-directed motility of arms and hands |
| Hold 1 object (1) | 4—Held one object with one hand |
| Hold 2 objects (2) | 5—Held an object in each hand by grasping the second object without dropping the first |
| Hold 3 objects (3) | 6—Held two objects in one hand and acquired a third without dropping one of the others |
Growth models from 6 to 12 months for holding.
| Conditional Growth | Unconditional Growth | |
|---|---|---|
| Coefficient | Coefficient | |
| Fixed Effects | ||
| Intercept (γ00) | −1.19 ** | −1.05 ** |
| Age (β10) | −1.02 ** | −1.33 ** |
| Age2 (β20) | 0.03 | 0.10 ** |
| Left (β01) | 0.25 | − |
| Left * Age (β11) | −0.82 ** | − |
| Left * Age2 (β21) | 0.17 ** | − |
| Right (β02) | 0.24 | − |
| Right * Age (β12) | −0.54 ** | − |
| Right * Age2 (γ22) | 0.11 ** | − |
| δ(1) | 2.46 ** | 2.45 ** |
| δ(2) | 8.62 ** | 8.52 ** |
| Random Effects | ||
| Intercept (r0i) | 0.82 ** | 0.79 ** |
Note: The unconditional growth is the model that estimates the expected change over time in holding. The conditional growth is the model that examines whether laterality influences changes in the holding patterns. * p < 0.05. ** p < 0.01.
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; Nelson, Eliza L 3
1 School of Kinesiology, Louisiana State University, Baton Rouge, LA 70802, USA;
2 Department of Psychology, University of North Carolina Greensboro, Greensboro, NC 27412, USA;
3 Department of Psychology, Florida International University, Miami, FL 33199, USA