Objectively measured daytime sleepiness predicts weight change among adults: Findings from the Wisconsin Sleep Cohort Study

OBJECTIVE

Body mass index (BMI) trajectories are associated with night-time sleep, but it is not clear how they relate to daytime sleepiness in population data. This study aimed to examine longitudinal associations between levels and changes in daytime sleepiness and BMI trajectories among men and women.

METHODS

We estimated growth curve models among 827 participants in the Wisconsin Sleep Cohort Study (mean [sd] age = 55.2 [8.0] years at baseline). The outcome variable was BMI (kg/m2) and the key predictor was daytime sleepiness measured by Multiple Sleep Latency Test (MSLT) scores. Covariates included demographics, health behaviors, retirement status, stimulant use, and depressive symptoms. In sensitivity analyses, we evaluated the potential effects of cardiovascular disease, shift work status, and sleep apnea on the robustness of sleepiness and BMI associations.

RESULTS

At the between-person level, men who were sleepier had higher BMI levels. At the within-person level, age moderated the positive association between sleepiness and BMI among women. Specifically, young women who became sleepier over time gained more BMI than older women with comparable increases in sleepiness. Furthermore, while BMI tended to increase with age among women, BMI trajectories were steeper among sleepy women than among well-rested women, who experienced less increase in BMI over time.

CONCLUSION

The study suggested that levels and changes in daytime sleepiness as objectively measured by MSLT scores are associated with body mass among adults.

Self-Perceived Infertility is Not Always Associated with Having Fewer Children: Evidence from German Panel Data

Proximate determinants theory considers infertility rates a risk factor for lower fertility rates, but the assumption that people who perceive infertility will have fewer children has not been tested. This study investigates the association of self-perceived infertility with the number of children people have had after 11 years. Infertility implies reduced chances of conception (rather than sterility), but people do not always consistently perceive infertility over time. If people who think they are infertile at one time can later report no infertility, then does self-perceived infertility necessarily lead to having fewer children? We answer this question by analyzing 11 waves of the German family panel (pairfam) data using negative binomial growth curve models for eight core demographic subgroups created by combinations of gender (men/women), parity (0/1+children), and initial age groups (25-27 and 35-37). Those who repeatedly perceived themselves to be infertile (three times or more) had fewer children than those who perceived themselves to be infertile once or twice in only four of eight gender by initial parity by age groups. Only in four groups did people who perceived themselves to be infertile once or twice have fewer children than those who never perceived themselves to be infertile in both the unadjusted and adjusted models. Thus, self-perceived infertility does not necessarily result in fewer children. Rather, the association depends upon life course context and gender.

Religiosity and trajectories of lifetime fertility intentions - Evidence from a German panel study

Much of the literature on fertility intentions has shown that they are broadly predictive of fertility behaviour. Fertility intentions tend to change over a person's life. How religiosity affects these changes over time has rarely been the subject of investigation. In this paper, we focus on whether and how religiosity affects trajectories of lifetime fertility intentions. Specifically, we examine whether highly religious people start with higher fertility intentions and are more likely to sustain them during their life course compared to their less religious counterparts. We apply random and fixed effects growth curve models to data from the German family panel pairfam, using a sample of 6214 women and 5802 men aged 14-46. We find that religiosity mainly contributes to explain the starting level at teenage years but not the trajectories of lifetime fertility intentions as people get older. Highly religious people start with higher intentions than less religious people. However, similarly to less religious people they experience a decline in their fertility intentions with age. This study demonstrates that religiosity is an important variable in research on fertility intentions but with changing relevance over the life course.

Dementia and Cognitive Decline in Older Adulthood: Are Agricultural Workers at Greater Risk?

OBJECTIVES

To examine whether long-term exposure to agricultural work is associated with dementia prevalence and the rate of cognitive change in older adulthood.

METHOD

We employed data from the Health and Retirement Study (1998-2014). Multiple logistic regression was used to determine whether a longest-held job in the agricultural sector was associated with differences in dementia prevalence. We examined if hearing impairment, depression, and physical health indicators mediated the relationship between agricultural work and cognitive functioning. Subgroup analyses were done by age, retirement status, job tenure, and cognitive domain. We employed growth curve models to investigate implications of agricultural work on age trajectories of cognitive functioning.

RESULTS

Longest-held job in agriculture, fishing, and forestry (AFF) was associated with 46% greater odds of having dementia. The relationship between AFF exposure and cognitive functioning was not mediated by hearing impairment, depression, or physical health indicators. Results were stronger among younger and retired older adults as well as those with extensive job tenure. AFF exposure was associated with lower scores in working memory and attention and processing speed. Growth curve models indicated that while agricultural work exposure was associated with lower initial levels of cognitive functioning, over time, the pattern reversed with individuals in non-AFF jobs, showing more accelerated cognitive decline.

DISCUSSION

Consistent with European studies, results from the United States also demonstrate a higher prevalence of dementia among agricultural workers. The cognitive reserve framework may explain the seemingly paradoxical result on age patterning of cognitive performance across older adults with different work histories.

Dataset on growth curves of Boer goats fitted by ten non-linear functions

Data on the description of growth of female Boer goats from the Mexican national breeding flock are presented. Goat meat is highly appreciated for the preparation of traditional dishes of Mexican cuisine, and its demand is on the rise. Boer goats are of relatively recent arrival in Mexico and the size of the performance-recorded flock has been increasing steadily in the last ten years. Repeated measures of body weight at different ages from birth to adulthood of Boer goats are scarce. When available, such data can be used to describe the growth pattern and the meat production potential of goat meat breeds such as the Boer. This paper presents data on estimators of growth curve parameters, plots of average predicted growth curves, plots of residuals on age, and data on goodness of fit statistics of ten non-linear functions fitted to describe the growth curve of Boer goats.

Longitudinal modeling in developmental neuroimaging research: Common challenges, and solutions from developmental psychology

Hypotheses about change over time are central to informing our understanding of development. Developmental neuroscience is at critical juncture: although the majority of longitudinal imaging studies have observations with two time points, researchers are increasingly obtaining three or more observations of the same individuals. The goals of the proposed manuscript are to draw upon the long history of methodological and applied literature on longitudinal statistical models to summarize common problems and issues that arise in their use. We also provide suggestions and solutions to improve the design, analysis and interpretation of longitudinal data, and discuss the importance of matching the theory of change with the appropriate statistical model used to test the theory. Researchers should articulate a clear theory of change and to design studies to capture that change and use appropriately sensitive measures to assess that change during development. Simulated data are used to demonstrate several common analytic approaches to longitudinal analyses. We provide the code for our simulations and figures in an online supplement to aid researchers in exploring and plotting their data. We provide brief examples of best practices for reporting such models. Finally, we clarify common misunderstandings in the application and interpretation of these analytic approaches.

Racial variation in the effect of motherhood on women's employment: Temporary or enduring effect?

Part of the motherhood wage penalty results from mothers' loss of work experience, yet little research has investigated whether this loss is temporary or accumulates over time. Using growth curve models and data from the National Longitudinal Study of Youth (1979), I examine the extent to which motherhood reduces work experience over the life course among White, Black, and Hispanic women. Results indicate that motherhood slows the accretion of experience in full-time work for all racial-ethnic groups, having an enduring effect on women's employment. The effect is stronger among Whites and mothers with two or more children, remaining sizeable as women approach retirement age. By age 50, White and Hispanic mothers with two or more children exhibit between two to seven fewer years of experience in full-time employment. Among Blacks, only mothers with three or more children experience a significant reduction, averaging five fewer years of experience in full-time work.

Development of hot and cool executive functions in middle childhood: Three-year growth curves of decision making and working memory updating

Although middle childhood is an important period for the development of hot and cool executive functions (EFs), longitudinal studies investigating trajectories of childhood EF development are still limited and little is known about predictors for individual developmental trajectories. The current study examined the development of two typical facets of cool and hot EFs over a 3-year period during middle childhood, comparing a younger cohort (6- and 7-year-olds at the first wave [T1]; n = 621) and an older cohort (8- and 9-year-olds at T1; n = 975) of children. "Cool" working memory updating (WM) was assessed using a backward digit span task, and "hot" decision making (DM) was assessed using a child variant of the Iowa Gambling Task. Linear latent growth curve analyses revealed evidence for developmental growth as well as interindividual variance in the initial level and rate of change in both EF facets. Initial level of WM was positively associated with age (both between and within cohorts), socioeconomic status, verbal ability, and processing speed, whereas initial levels of DM were, in addition to a (potentially age-related) cohort effect, exclusively predicted by gender, with boys outperforming girls. None of the variables predicted the rate of change, that is, the developmental trajectories. However, younger children, as compared with older children, had slightly steeper WM growth curves over time, hinting at a leveling off in the development of WM during middle childhood. In sum, these data add important evidence to the understanding of hot and cool EF development during middle childhood.

The age pattern of social inequalities in health at older ages: are common measures of socio-economic status interchangeable?

OBJECTIVES

Social inequalities in health have been largely documented in social science research. Members of the most disadvantaged groups experience worse health and higher mortality from birth throughout adulthood. However, it is not clear whether this association persists at older ages. Some studies have found a narrowing of the social gradient in health, at least when 'traditional' measures of socio-economic status (SES)-income, education, and occupation-are used. The main goal of the article is to highlight similarities and discrepancies in the age trend of social inequalities in health that arise when multiple measures of SES are considered.

STUDY DESIGN

The present study uses a longitudinal sample of over 7000 individuals age 50+ from the Survey of Health, Ageing, and Retirement in Europe to examine the age trend of social inequalities in health.

METHODS

By using growth curve models, individual trajectories of self-rated health and physical functioning were analyzed. SES is measured through wealth, income, and education.

RESULTS

The findings show that for both health outcomes, the choice of the indicator of SES is very consequential, as the age trend of social inequalities in health is substantially different for different measures of SES.

CONCLUSION

Using multiple measures of SES is recommended, as using only one measure would give only a partial account of the age trend of social inequalities in health. In particular, wealth seems to better capture individual's socio-economic position, as it is able to detect health gradients even where education and income fail to do so.

Evolution of model specific relative growth rate: Its genesis and performance over Fisher's growth rates

Growth curve models play an instrumental role to quantify the growth of biological processes and have immense practical applications across disciplines. In the modelling approach, the absolute growth rate and relative growth rate (RGR) are two most commonly used measures of growth rates. RGR is empirically estimated by Fisher (1921) assuming exponential growth between two consecutive time points and remains invariant under any choice of the underlying growth model. In this article, we propose a new measure of RGR, called modified RGR, which is sensitive to the choice of underlying growth law. The mathematical form of the growth equations are utilized to develop the formula for model dependent growth rates and can be easily computed for commonly used growth models. We compare the efficiency of Fisher's measure of RGR and modified RGR to infer the true growth profile. To achieve this, we develop a goodness of fit testing procedure using Gompertz model as a test bed. The relative efficiency of the two rate measures is compared by generating power curves of the goodness of fit testing procedure. The asymptotic distributions of the associated test statistics are elaborately studied under Gompertz set up. The simulation experiment shows that the proposed formula has better discriminatory power than the existing one in identifying the true profile. The claim is also verified using existing real data set on fish growth. An algorithm for the model selection mechanism is also proposed based on the modified RGR and is generalized for some commonly used other growth models. The proposed methodology may serve as a valuable tool in growth studies in different research areas.

Sum Scores in Twin Growth Curve Models: Practicality Versus Bias

To study behavioral or psychiatric phenotypes, multiple indices of the behavior or disorder are often collected that are thought to best reflect the phenotype. Combining these items into a single score (e.g. a sum score) is a simple and practical approach for modeling such data, but this simplicity can come at a cost in longitudinal studies, where the relevance of individual items often changes as a function of age. Such changes violate the assumptions of longitudinal measurement invariance (MI), and this violation has the potential to obfuscate the interpretation of the results of latent growth models fit to sum scores. The objectives of this study are (1) to investigate the extent to which violations of longitudinal MI lead to bias in parameter estimates of the average growth curve trajectory, and (2) whether absence of MI affects estimates of the heritability of these growth curve parameters. To this end, we analytically derive the bias in the estimated means and variances of the latent growth factors fit to sum scores when the assumption of longitudinal MI is violated. This bias is further quantified via Monte Carlo simulation, and is illustrated in an empirical analysis of aggression in children aged 3-12 years. These analyses show that measurement non-invariance across age can indeed bias growth curve mean and variance estimates, and our quantification of this bias permits researchers to weigh the costs of using a simple sum score in longitudinal studies. Simulation results indicate that the genetic variance decomposition of growth factors is, however, not biased due to measurement non-invariance across age, provided the phenotype is measurement invariant across birth-order and zygosity in twins.

Modeling error distributions of growth curve models through Bayesian methods

Growth curve models are widely used in social and behavioral sciences. However, typical growth curve models often assume that the errors are normally distributed although non-normal data may be even more common than normal data. In order to avoid possible statistical inference problems in blindly assuming normality, a general Bayesian framework is proposed to flexibly model normal and non-normal data through the explicit specification of the error distributions. A simulation study shows when the distribution of the error is correctly specified, one can avoid the loss in the efficiency of standard error estimates. A real example on the analysis of mathematical ability growth data from the Early Childhood Longitudinal Study, Kindergarten Class of 1998-99 is used to show the application of the proposed methods. Instructions and code on how to conduct growth curve analysis with both normal and non-normal error distributions using the the MCMC procedure of SAS are provided.

Functional limitations and depressive symptoms: a longitudinal analysis of older Mexican American couples

OBJECTIVES

This study extends previous work on longitudinal patterns of spousal associations between functional impairments and psychological well-being in older couples in 3 important ways: By examining Mexican Americans, by considering a broader range of functional limitations, and by assessing the role of health status, social integration, and socioeconomic resources in these associations.

METHOD

Drawing on data from 6 waves of the Hispanic Established Population for the Epidemiologic Study of the Elderly (1993-2007), we employed growth curve models to investigate the implications of the spouse's functional limitations for the respondent's age trajectories of depressive symptoms in older Mexican American couples. Models were run separately for husbands and wives.

RESULTS

The spouse's functional limitations were associated with higher levels of depressive symptoms in the respondent. Personal resources can both ameliorate and intensify the adverse implications of the spouse's functional limitations for the respondent's depressive symptomatology. The interplay among these factors can vary by gender and the type of the spouse's functional impairment.

DISCUSSION

Future studies would benefit by examining caregiving patterns in older couples, by distinguishing between different dimensions of social support available to them, and by considering changes in couples' marital quality and social ties over time.

Exploring the impact of the 2008 global food crisis on food security among vulnerable households in rural South Africa

Recurring food crises endanger the livelihoods of millions of households in developing countries around the globe. Owing to the importance of this issue, we explore recent changes in food security between the years 2004 and 2010 in a rural district in Northeastern South Africa. Our study window spans the time of the 2008 global food crises and allows the investigation of its impacts on rural South African populations. Grounded in the sustainable livelihood framework, we examine differences in food security trajectories among vulnerable sub populations. A unique panel data set of 8,147 households, provided by the Agincourt Health and Demographic Surveillance System (Agincourt HDSS), allows us to employ a longitudinal multilevel modeling approach to estimate adjusted growth curves for the differential change in food security across time. We observe an overall improvement in food security that leveled off after 2008, most likely resulting from the global food crisis. In addition, we discover significant differences in food security trajectories for various sub populations. For example, female-headed households and those living in areas with better access to natural resources differentially improved their food security situation, compared to male-headed households and those households with lower levels of natural resource access. However, former Mozambican refugees witnessed a decline in food security. Therefore, poverty alleviation programs for the Agincourt region should work to improve the food security of vulnerable households, such as former Mozambican refugees.

Trajectories of growth in body mass index across childhood: Associations with maternal and paternal employment

Research links mothers' employment to higher body mass index (BMI), a measure of weight-for-height, among their children. However, how maternal employment patterns relate to their children's BMI trajectories, and the role that fathers' employment plays in when and at what rate children grow, remain unclear. With data on children from 2 to 15 years of age living in two-parent families from the U.S. NICHD's Study of Early Child Care and Youth Development (N = 1107), individual growth models are used to describe American children's BMI trajectories as predicted by maternal and paternal employment characteristics. Results indicate that, by age 15, children's BMIs are, on average, nearly one-half of a standard deviation above recommended levels, and the majority of growth occurs during the preschool period. The duration of maternal employment, and combined measures of maternal and paternal employment duration, are both associated with higher child BMI across childhood. Associations are small but cumulative. Notably, the association between the duration of time children lived in dual-earner families and child BMI is larger than that between maternal employment duration alone and children's BMI, which is strongest during the preschool period. Combined measures of maternal and paternal employment intensity, defined as the number of periods both parents worked 35 or more hours per week, are associated with higher child BMI during the preschool period only. Findings highlight the importance of taking into account both parents' employment characteristics in investigating children's physical development.

Correlates of individual, and age-related, differences in short-term learning

Latent growth models were applied to data on multitrial verbal and spatial learning tasks from two independent studies. Although significant individual differences in both initial level of performance and subsequent learning were found in both tasks, age differences were found only in mean initial level, and not in mean learning. In neither task was fluid or crystallized intelligence associated with learning. Although there were moderate correlations among the level parameters across the verbal and spatial tasks, the learning parameters were not significantly correlated with one another across task modalities. These results are inconsistent with the existence of a general (e.g., material-independent) learning ability.