Cognitive deficits are highly prevalent in multiple sclerosis (MS) and have a negative impact on daily life. Impairments in information processing speed are among the most commonly reported deficits in MS and are generally assessed by evaluating mean-level performance on time-limited tests. However, this approach to assessing performance ignores potential within-subject differences that may be useful for characterizing cognitive difficulties in MS. An alternative method of measuring performance on timed cognitive tasks is to examine the degree of within-subject variability, termed intraindividual variability (IIV). IIV provides information about the characteristics of an individual’s performance and may provide novel information about cognitive functioning in MS and other neurodegenerative disorders. The research presented in this dissertation examined IIV in performance as an indicator of cognitive functioning in persons with MS and explored the relations of performance variability to measures of neuronal connectivity derived from resting state functional magnetic resonance imaging (rsfMRI). Individuals with MS were found to be both slower and more variable on tests of information processing speed and attention. This variability was observed even when controlling for sensorimotor confounds and other systematic variables that may influence variability, such as practice and learning effects. IIV in performance was found to better distinguish MS patients from matched groups of healthy control subjects when compared to common clinical measures of cognitive performance or average response speed. These differences in IIV were also found consistently across six monthly assessments in a group with MS who remained clinically stable over this period. This stability in IIV suggests its feasibility as a measure of changes in longitudinal cognitive or clinical status. Using rsfMRI, greater stability in performance (i.e., lower IIV) was associated with greater functional connectivity between frontal lobe regions (i.e., ventral medial prefrontal cortex and frontal pole) in persons with MS. This increased connectivity appears to represent potential compensatory processes within mildly affected MS individuals. Together the findings demonstrate that IIV is an important characteristic of cognitive performance that may provide new insights into the cognitive deficits present in MS.

Investigators have shown that elephants exhibit consistent individual differences in behavior by rating elephants using personality adjectives. These adjectives, however, are not based on pre-defined measurements of the behaviors performed. Instead, they are based on the observers’ interpretations of an animal’s behavioral patterns, therefore making them subject to observer bias. Furthermore, elephants have a capacity for learning; thus, they may alter their behavioral patterns over time. This behavioral plasticity in itself might be a way of measuring consistent behavioral differences among individuals. With this in mind, I approached elephant personality as a multivariate problem. I used behavioral observations collected from female elephants in Addo Elephant National Park, South Africa. Instead of grouping behaviors into subjective categories prior to analysis (as is often done in studies of elephant behavior), I used ordination methods to determine which correlations among behaviors were important for defining personality. Ordination methods were performed on matrices of the behavior data set and on subsets of behaviors for each age class. I calculated the angular differences among major axes of covariation from the ordinations of subsets to determine if the behaviors that defined personalities differed by age class. I also defined personalities by centroids (in multidimensional space) for non-metric multidimensional scaling (NMDS) scores of each individual and dispersion of NMDS scores for each individual as a measure of behavioral plasticity. I analyzed the effects of plasticity and age on personality of individual elephants using a non-parametric multivariate analysis of variance. Major axes of covariation were not well defined and therefore not useful in describing differences among groups. The interaction of age and behavioral plasticity did have a significant effect on the personalities of individuals as defined by ordination centroid scores. This suggests that incorporating plasticity may be a helpful measurement in quantifying consistent behavioral differences among individuals.

To characterize the relationship between dispersion-based intra-individual variability (IIVd) in neuropsychological test performance and brain volume among HIV seropositive and seronegative men and to determine the effects of cardiovascular risk and HIV infection on this relationship. Magnetic Resonance Imaging (MRI) was used to acquire high-resolution neuroanatomic data from 147 men age 50 and over, including 80 HIV seropositive (HIV+) and 67 seronegative controls (HIV-) in this cross-sectional cohort study. Voxel Based Morphometry was used to derive volumetric measurements at the level of the individual voxel. These brain structure maps were analyzed using Statistical Parametric Mapping (SPM2). IIVd was measured by computing intra-individual standard deviations (ISD’s) from the standardized performance scores of five neuropsychological tests: Wechsler Memory Scale-III Visual Reproduction I and II, Logical Memory I and II, Wechsler Adult Intelligence Scale-III Letter Number Sequencing. Total gray matter (GM) volume was inversely associated with IIVd. Among all subjects, IIVd -related GM atrophy was observed primarily in: 1) the inferior frontal gyrus bilaterally, the left inferior temporal gyrus extending to the supramarginal gyrus, spanning the lateral sulcus; 2) the right superior parietal lobule and intraparietal sulcus; and, 3) dorsal/ventral regions of the posterior section of the transverse temporal gyrus. HIV status, biological, and cardiovascular disease (CVD) variables were not linked to IIVd -related GM atrophy. IIVd in neuropsychological test performance may be a sensitive marker of cortical integrity in older adults, regardless of HIV infection status or CVD risk factors, and degree of intra-individual variability links with volume loss in specific cortical regions; independent of mean-level performance on neuropsychological tests.

Insufficient sleep duration is associated with poor physical and mental health outcomes, and many Americans report that they are not meeting sleep duration recommendations. Many individuals choose to restrict their own sleep, yet little is known about the source of this sleep deficit. Recent research efforts have used the Theory of Planned Behavior (TPB) to predict sleep health behavior. However, this research is limited in that it fails to measure volitional sleep behavior and focuses exclusively on between-person differences. This study addressed these limitations by using an intensive longitudinal design to test how constructs of the TPB relate to nightly sleep opportunity. Healthy college students (N=79) completed a week long study in which they completed 4 ecological momentary assessment signals per day that measured their attitudes, perceived norms, perceived behavioral control (PBC), and intentions relating to their nocturnal sleep opportunity. Participants wore an actiwatch each night of the study to measure their sleep opportunity. Analyses revealed between- and within-day variability of attitudes, perceived norms, PBC, and intentions. Further, there were significant between- and within-day trajectories of these constructs. Mixed linear models demonstrated that both intentions and PBC were significant predictors of subsequent sleep opportunity, and that PBC was the strongest predictor of future intentions. The between-and within-day patterns of these constructs highlight important considerations for their measurement, and provide insight into the potential refinement of sleep promotion efforts. Results also demonstrate that within-person changes in PBC and intentions predict subsequent sleep opportunity, demonstrating the need for a daily framework when using the TPB to predict sleep health behavior.

AN ABSTRACT OF THE DISSERTATION OFKaleigh C. Best, for the Doctor of Philosophy degree in Anthropology, presented on May 12, 2023, at Southern Illinois University Carbondale. TITLE: AN INVESTIGATION INTO INTRAINDIVIDUAL VARIATION IN HUMAN SKELETAL SEXUAL TRAIT MORPHOLOGYMAJOR PROFESSOR: Dr. Izumi ShimadaSex estimation is the most vital component of the biological profile assessment as several other subsequent analyses across biological anthropology rely on the correct identification of biological sex. This is especially important in times of commingling, fragmentary and partial remains, in inconsistent sex estimation, or in studies of sex estimation variation. While a multitude of studies have contributed to the documentation of similarities and differences in the morphological expression of a specific trait and how well this trait can be used to classify the sex of an unknown individual, how the expression of these morphological traits relate to each other within the individual has been poorly studied. Further, although several intrinsic and extrinsic factors are known to contribute to sexual trait expression, how they interact to produce morphology, and how that morphology may change in response to different biological conditions is not well understood. This dissertation utilizes 3D scans of 235 individuals collected from the University of Tennessee Donated Skeletal Collection (UTK) and the Robert J. Terry Osteological Collection (Terry) to examine sexual trait expression across the cranium, mandible, os coxae, sacrum and scapula. Specifically, it evaluates: 1) if sexual trait expression is correlated between skeletal regions including localized anatomical regions, through matrix correlation and Pearson correlation tests, 2) if age affects morphological sexual trait expression through M/ANOVA and if so, how and when these changes occur using ANOVA and Tukey post-hoc tests, 3) if the magnitude and pattern of sexual trait expression varies between populations through M/ANOVA and matrix correlations, 4) using discriminate function analyses, how well geometric morphometric analyses capture sexual trait expression compared to published rates, and 5) through M/ANOVA and discriminant function analyses, which intraindividual skeletal regions are the most accurate in sex classification analyses compared to others. For each skeletal element, landmarks were placed on each scan and sexual trait expression and an intra observer placement study was conducted. The landmark configurations were then subjected to a generalized Procrustes analysis, variance- covariance matrices were generated, and a principal component (PC) analyses in MorphoJ for combined sex data, derived male and females from combined sex data, and for independently input males and females. PCs were visually interpreted and subjected to M/ANOVA for three age groups: 18-29, 30-45, and 45 and older. PCs were then subjected to stepwise linear discriminant function analyses with leave- one-out cross validation. The first five PCs for each element were then utilized for a matrix correlation test and evaluated using a strength of correlation test (Chan et al 2003) and for significance using a Pearson’s correlation test. Results indicate that for intraobserver error is low, with scores being mostly considered excellent in reliability. For UTK data, shape changes associated with sexual trait classification were found for each of the five elements. Within these tests, many influential regions were known to sex estimation analyses, but other, such as the basicranium are currently underutilized. M/ANOVA revealed that sex was significant for all five skeletal elements, with some contributions from age and ancestry impacting sexual trait expression, however these interactions were not within a consistent pattern. Through the results of the discriminant function analyses, it appears some skeletal elements, such as the os coxae, scored higher correct classification rates than those reported in the literature, while others were on par or below reported rates. The os coxae and regions around it, were the best elements for correct sex classification accuracy, with the scapula rated as the worst. In the Terry data, sexual trait related shape changes are presented for each of the skeletal elements studied. Known morphological regions, as well as some more subtle ones, were found to be influential. M/ANOVA revealed that all elements, but the scapula had statistically significant shape changes present for sex estimation. Age and ancestry appear to contribute minimally to sexual trait morphology, but again, not in a way that is consistent, either within the collection or similarly to UTK. By examining the discriminant function analyses, some correct sex-estimation rates were on higher or on par with those reported in the literature, but others, such as the scapula were lower. This may be due the size being a larger contributor for sex estimation than shape, and the geometric morphometric analyses minimizing its effects in these analyses. Also using these tests, the os coxae was selected as the best element for high levels of sex classification accuracy, but which element was next best was different than the UTK results. This likely indicates biocultural influences are affecting sexual trait morphology and should be considered in future analyses. Significant inter-element correlations were detected in both skeletal collections, with UTK having more pairings than Terry. The most common and most statistically significant inter-element correlation across collections was between the cranium-mandible, which may be reflective of the integrated nature of these two elements. Different element correlations existed between each collection, with no clear pattern in pairings present in both collections, which may suggest that biocultural influences are impacting sexual trait expression. However, in both collections, females had more overall correlations, and more statistically significant correlations than males, although there was less of a difference between sexes in Terry than in UTK. This result may be due to circulating hormones present during puberty, which may be contributing to more correlation between elements in females and not males. It also suggests that once again, biocultural differences between the Terry and UTK collections may be impacting the magnitude and patterns of correlation. This research highlights the complex dynamic nature of sexual trait expression in bone in relation to several intrinsic and extrinsic factors. It supports the idea that sexual trait expression is not concordant between skeletal regions among the same individual and that these inter-element correlations appear to be different between skeletal collections. Likewise, it suggests that age and ancestry may minimally be affecting sexual trait expression, but not in a consistent way across skeletal elements or between collections. It also suggests that while geometric morphometrics may help in capturing shape variation, it does not always translate to higher correct classification rates. This research also supports previous literature that the os coxae is the best element for sex estimation, although is not clear on what element is next best. Future studies in sexual trait expression should account for biocultural influences, as this research suggests that sexual trait expression is influenced by poorly understood intrinsic and extrinsic factors.

Submitted by Jean Medeiros (jeanletras@uepb.edu.br) on 2016-05-17T12:56:17Z No. of bitstreams: 1 PDF - Maria Rita Nascimento Duarte.pdf: 2708130 bytes, checksum: 83ff7c3f4dd9638f2a12090069b8a8e8 (MD5) / Approved for entry into archive by Secta BC (secta.csu.bc@uepb.edu.br) on 2016-07-21T20:42:27Z (GMT) No. of bitstreams: 1 PDF - Maria Rita Nascimento Duarte.pdf: 2708130 bytes, checksum: 83ff7c3f4dd9638f2a12090069b8a8e8 (MD5) / Made available in DSpace on 2016-07-21T20:42:27Z (GMT). No. of bitstreams: 1 PDF - Maria Rita Nascimento Duarte.pdf: 2708130 bytes, checksum: 83ff7c3f4dd9638f2a12090069b8a8e8 (MD5) Previous issue date: 2016-02-02 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES / In the Neotropical zone the generalist habit is common among fish populations, especially in semi-arid rivers. In these systems, where the availability of food items varies spatially and temporally, it is expected that a more generalist strategy dominates. However, populations classified as generalist may actually be composed of relatively specialist individuals. This study aims at quantifying the use of food resources and understanding the individual contribution to population niche of Hoplias malabaricus, Geophagus brasiliensis and Prochilodus brevis. Four collections were performed at four sites along the Ipanema (PE) River, in April and July 2007 (wet season) and October 2007 and January 2008 (dry season). Sampling was performed at daylight, using gillnets, manual trawls and cast nets. The specimens collected were fixed in 4% formalin and preserved in 70% ethanol. The diet was quantified using the indirect volumetric method, frequency of occurrence and numeric frequency. Spatial patterns of variation in food items between species were evaluated using the Nonmetric Multidimensional Scaling (NMS). The Multi-Response Permutation Procedure (MRPP) was used to test the significance of differences in diet composition between species, sites and sampling periods. The degree of specialization of each individual was calculated using a proportional similarity index. The amplitude and niche overlap ratios were calculated using the Levins and Bolnick indexes. The graphical analysis of Amundsen was performed in order to correlate the diet composition with the selection of specific prey. The species showed a pattern of distribution and segregation in diet between the sampling sites. The MRPP showed that there were significant differences in the diet composition between species (MRPP, A= 0, 61; p= 0, 00). Geophagus brasiliensis specie showed significant differences between site 1 and site 4 (MRPP, A = 0.14; p = 0.03) and site 2 and site 4 (MRPP, A = 0.16 p = 0, 03), the Prochilodus brevis specie between site 1 and site 3 (MRPP, A = 0.28 p = 0.02) and site 1 and site 4 (MRPP, A = 0.11; p = 0.04). The three species present individual specialization in all sampling sites and in both periods of the hydrological cycle. The species Hoplias malabaricus has a population with most experts individuals, with rest of fish as the dominant item, however, with other individuals in the population consuming a high abundance of specific prey. However Geophagus brasiliensis presented a generalist population, with individuals of the population consuming a variety of items, displaying a relatively high component within phenotype. Prochilodus brevis showed a population with varied feeding strategy, presenting degrees of specialization with the contribution of Cyclotela item and generalization, where food items were consumed by more than half of the population. It was observed that individual specialization is highly correlated with niche overlap, indicating that the smaller the overlap the larger the degree of individual specialization. It is important to emphasize that the greatest degree of individual specialization was observed where there was a greater availability of food resources consumed by the populations. / Na região neotropical o hábito generalista é comum em populações de peixes, especialmente em rios do semiárido. Nesses sistemas onde a distribuição dos itens alimentares varia espacial e temporalmente, espera-se que a estratégia generalista predomine entre as espécies. Este estudo objetiva quantificar a utilização de recursos alimentares e compreender a contribuição individual para o nicho populacional em três espécies de peixes, Hoplias malabaricus, Geophagus brasiliensis e Prochilodus brevis. Foram realizadas coletas em quatro pontos ao longo do rio Ipanema (PE) nos meses de abril e julho de 2007 (período chuvoso) e outubro de 2007 e janeiro de 2008 (período seco). As amostragens foram feitas durante o dia, usando redes de arrasto, espera e tarrafa. Os espécimes coletados foram fixados em formol 4% e preservados em álcool 70%. A dieta foi quantificada usando o método volumétrico indireto, frequência de ocorrência e a frequência numérica. Padrões espaciais de variação de itens alimentares para as espécies foram avaliados por meio Escalonamento Multidimensional Não- Métrico (NMS). O Procedimento de Permutações Múltiplas (MRPP) foi utilizado para testar diferenças na composição da dieta entre as espécies, pontos e períodos de amostragem. O grau de especialização de cada indivíduo foi calculado utilizando um índice de similaridade proporcional. A amplitude e sobreposição de nicho foram calculadas utilizando os índices de Levins e Bolnick. Foi realizada a análise gráfica de Amundsen a fim de relacionar a composição da dieta com a seleção de presas específicas. O MRPP mostrou que houve diferenças significativas na composição da dieta entre as espécies (A = 0,61; p = 0,005). Entre os pontos de coleta a espécie Geophagus brasiliensis apresentou diferenças significativas entre o ponto 1 e ponto 4 (MRPP, A = 0,14 e p = 0,03) e ponto 2 e ponto 4 (MRPP, A = 0,16 e p = 0,03), a espécie Prochilodus brevis entre o ponto 1 e ponto 3 (MRPP, A = 0,28 e p = 0,02) e ponto1 e ponto 4 (MRPP, A = 0,11 e p = 0,04). As três espécies apresentaram especialização individual em todos os pontos de coletas e em ambos os períodos de regime hidrológico. A espécie Hoplias malabaricus apresentou uma população com a maioria dos indivíduos especialistas, apresentando resto de peixe como item dominante, porém, com outros indivíduos da população consumindo uma alta abundância de presas especificas. Geophagus brasiliensis apresentou uma população generalista, com indivíduos da população consumindo uma variedade de itens, exibindo um componente relativamente elevado dentre- fenótipo. Prochilodus. brevis mostrou uma população com estratégia de alimentação variada, apresentando graus de especialização com a contribuição do item alimentar Cyclotela e generalização, onde os itens alimentares tinham sido consumidos por mais da metade da população. Observou-se que a especialização individual está correlacionada com a sobreposição de nicho, indicando que, quanto menor sobreposição maior é o grau de especialização individual. É importante ressaltar que o maior grau de especialização individual foi observado onde houve uma maior riqueza de recursos alimentares consumidos pelas populações.

Traditionally, reaction time (RT) was conceived of as an average speed of a number of responses made by an individual, or mean RT. Increasingly, however, intraindividual variability in reaction time (RT IIV) – the consistency of responses by a single person across trials – is used as an additional or even alternative measure. RT IIV is often found to be elevated in a number of conditions that affect the central nervous system functioning, such as traumatic brain injury or neurodegenerative diseases. It can predict change in cognitive performance in ageing, progression from normal ageing to mild cognitive impairment, and even death. Therefore, RT IIV may be of great practical importance. However, RT IIV and mean RT are correlated; therefore it is often problematic to draw conclusions about unique associations between these and other variables. One objective of the work presented in this thesis was to investigate determinants and correlates of simple and choice RT IIV and to test which associations may be accounted for by the individual differences in mean RT. The first investigation was concerned with age differences in RT IIV. Following a systematic review of literature, a series of meta-analyses demonstrated that older individuals (aged 60 years and above) have greater RT IIV than young or middle-aged adults in simple and choice RT tasks. The effects were reduced but still significant when RT IIV was adjusted for mean RT. The next study was a cross-sectional investigation of the associations between age and RT IIV, as well as of sex differences in RT IIV, across the lifespan in participants ranging in age from 4 to 75. Non-linear effects of age were found for RT IIV measures, such that variability decreased with age in children and increased with age in older adults. A novel finding from this study was that sex differences in RT IIV were present among adults but not children, suggesting that there might be an age threshold at which sexes diverge in their RT IIV trajectories. The results also indicated that findings regarding RT IIV may differ depending on the variability measure used (that is, whether and how mean RT is controlled). The second study on the same sample investigated variability on a trial-by-trial basis. Specifically, it tested the hypothesis that sex differences in variability are due to females being disproportionately slower at the first trial which inflates their overall RT IIV. This hypothesis was not supported. Another investigation used longitudinal data from the West of Scotland Twenty-07 study. Three cohorts of individuals aged approximately 15, 35 and 55, were followed up for 20 years and had RT data collected at four occasions. Analyses confirmed non-linear effects of age on RT IIV found in the earlier cross-sectional investigation. The final study investigated the effect of high altitude on RT IIV. It found that altitude-related increase in RT IIV is fully accounted for by general slowing of RT at high altitude. The overall pattern of results obtained from the investigations suggests that RT IIV increases with age in adults and that not all of the increase is due to general slowing. Moreover, the results show that sex differences in RT IIV are not uniform across the lifespan. Finally, whereas associations of RT IIV with some variables, for example age, are relatively robust to controlling for mean RT, others are fully attenuated by such practice.

<p>Elevated levels of anxiety are a common response to stressful competitive sports situations, are known to moderate athletic performance and are referred to as an unpleasant emotional state associated with perceptions of situational threat. The empirical studies in this dissertation considered primarily psychometric, methodological and conceptual issues of relevance for the study of anxiety and sports performance. In Study I, athletes were followed across a full competitive season to explore patterns of inter- and intra-individual variability of anxiety and self-confidence in relation to performance. The findings imply intra-individual anxiety and self-confidence variability to affect performance differently than the specific intensity level and are discussed in relation to more stable personality dispositions such as private self-consciousness. Study II evaluated the psychometric properties of the 27-item Competitive State Anxiety Inventory-2 (CSAI-2) and alternative versions of this scale. General support for a 17-item version (CSAI-2R) was found, but there are also psychometric limitations future research needs to resolve. Study III investigated assessment of intensity and directional ratings on single anxiety items with reference to the conceptualisation of anxiety symptoms as interpreted on a debilitative-facilitative continuum. The findings question the importance and rationale of assessing anxiety direction and revealed serious concerns with assessment procedures and statistical techniques applied in previous research. These concerns were also supported in Study IV, which explored athletes’ idiosyncratic experiences of debilitative and facilitative anxiety symptoms in terms of intensity and emotional valence. The findings are discussed and summarised in a model in order to increase conceptual clarity and provide implications for future research regarding anxiety and related emotional performance states.</p>

Elevated levels of anxiety are a common response to stressful competitive sports situations, are known to moderate athletic performance and are referred to as an unpleasant emotional state associated with perceptions of situational threat. The empirical studies in this dissertation considered primarily psychometric, methodological and conceptual issues of relevance for the study of anxiety and sports performance. In Study I, athletes were followed across a full competitive season to explore patterns of inter- and intra-individual variability of anxiety and self-confidence in relation to performance. The findings imply intra-individual anxiety and self-confidence variability to affect performance differently than the specific intensity level and are discussed in relation to more stable personality dispositions such as private self-consciousness. Study II evaluated the psychometric properties of the 27-item Competitive State Anxiety Inventory-2 (CSAI-2) and alternative versions of this scale. General support for a 17-item version (CSAI-2R) was found, but there are also psychometric limitations future research needs to resolve. Study III investigated assessment of intensity and directional ratings on single anxiety items with reference to the conceptualisation of anxiety symptoms as interpreted on a debilitative-facilitative continuum. The findings question the importance and rationale of assessing anxiety direction and revealed serious concerns with assessment procedures and statistical techniques applied in previous research. These concerns were also supported in Study IV, which explored athletes’ idiosyncratic experiences of debilitative and facilitative anxiety symptoms in terms of intensity and emotional valence. The findings are discussed and summarised in a model in order to increase conceptual clarity and provide implications for future research regarding anxiety and related emotional performance states.

Investigators have shown that elephants exhibit consistent individual differences in behavior by rating elephants using personality adjectives. These adjectives, however, are not based on pre-defined measurements of the behaviors performed. Instead, they are based on the observers’ interpretations of an animal’s behavioral patterns, therefore making them subject to observer bias. Furthermore, elephants have a capacity for learning; thus, they may alter their behavioral patterns over time. This behavioral plasticity in itself might be a way of measuring consistent behavioral differences among individuals. With this in mind, I approached elephant personality as a multivariate problem. I used behavioral observations collected from female elephants in Addo Elephant National Park, South Africa. Instead of grouping behaviors into subjective categories prior to analysis (as is often done in studies of elephant behavior), I used ordination methods to determine which correlations among behaviors were important for defining personality. Ordination methods were performed on matrices of the behavior data set and on subsets of behaviors for each age class. I calculated the angular differences among major axes of covariation from the ordinations of subsets to determine if the behaviors that defined personalities differed by age class. I also defined personalities by centroids (in multidimensional space) for non-metric multidimensional scaling (NMDS) scores of each individual and dispersion of NMDS scores for each individual as a measure of behavioral plasticity. I analyzed the effects of plasticity and age on personality of individual elephants using a non-parametric multivariate analysis of variance. Major axes of covariation were not well defined and therefore not useful in describing differences among groups. The interaction of age and behavioral plasticity did have a significant effect on the personalities of individuals as defined by ordination centroid scores. This suggests that incorporating plasticity may be a helpful measurement in quantifying consistent behavioral differences among individuals.