Tim A. Bruckner

To GEE or not to GEE: comparing population average and mixed models for estimating the associations between neighborhood risk factors and health.

Two modeling approaches are commonly used to estimate the associations between neighborhood characteristics and individual-level health outcomes in multilevel studies (subjects within neighborhoods). Random effects models (or mixed models) use maximum likelihood estimation. Population average models typically use a generalized estimating equation (GEE) approach. These methods are used in place of basic regression approaches because the health of residents in the same neighborhood may be correlated, thus violating independence assumptions made by traditional regression procedures. This violation is particularly relevant to estimates of the variability of estimates. Though the literature appears to favor the mixed-model approach, little theoretical guidance has been offered to justify this choice. In this paper, we review the assumptions behind the estimates and inference provided by these 2 approaches. We propose a perspective that treats regression models for what they are in most circumstances: reasonable approximations of some true underlying relationship. We argue in general that mixed models involve unverifiable assumptions on the data-generating distribution, which lead to potentially misleading estimates and biased inference. We conclude that the estimation-equation approach of population average models provides a more useful approximation of the truth.

The mental health workforce gap in low- and middle-income countries: a needs-based approach

OBJECTIVE To estimate the shortage of mental health professionals in low- and middle-income countries (LMICs). METHODS We used data from the World Health Organizations Assessment Instrument for Mental Health Systems (WHO-AIMS) from 58 LMICs, country-specific information on the burden of various mental disorders and a hypothetical core service delivery package to estimate how many psychiatrists, nurses and psychosocial care providers would be needed to provide mental health care to the total population of the countries studied. We focused on the following eight problems, to which WHO has attached priority: depression, schizophrenia, psychoses other than schizophrenia, suicide, epilepsy, dementia, disorders related to the use of alcohol and illicit drugs, and paediatric mental disorders. FINDINGS All low-income countries and 59% of the middle-income countries in our sample were found to have far fewer professionals than they need to deliver a core set of mental health interventions. The 58 LMICs sampled would need to increase their total mental health workforce by 239,000 full-time equivalent professionals to address the current shortage. CONCLUSION Country-specific policies are needed to overcome the large shortage of mental health-care staff and services throughout LMICs.

Secondary sex ratios and male lifespan: Damaged or culled cohorts

Population stressors reportedly reduce the human secondary sex ratio (i.e., the odds of a newborns being male) by, among other mechanisms, inducing the spontaneous abortion of males who would have been born live had mothers not been stressed. Controversy remains as to whether these abortions result from reduced maternal tolerance of males at the low end of a relatively constant distribution of survivability (i.e., the “culled cohort” explanation) or from shifts in the whole distribution of survivability such that more males fall below a relatively constant criterion of maternal tolerance for low survivability (i.e., the “damaged cohort” explanation). These alternatives make opposing predictions regarding the relationship between the secondary sex ratio and lifespan of male birth cohorts. We test the hypothesis that the secondary sex ratio among Swedish cohorts born in the years 1751 through 1912 predicts male cohort life expectancy at birth (i.e., realized lifespan). Our results support the culled cohort argument. We argue that these findings have implications for the basic literature concerned with temporal variation in the secondary sex ratio, for more applied work concerned with the fetal origins of adult health, and for public health surveillance.

Short-Term Effects of Air Pollution on Wheeze in Asthmatic Children in Fresno, California

Background Although studies have demonstrated that air pollution is associated with exacerbation of asthma symptoms in children with asthma, little is known about the susceptibility of subgroups, particularly those with atopy. Objective This study was designed to evaluate our a priori hypothesis that identifiable subgroups of asthmatic children are more likely to wheeze with exposure to ambient air pollution. Methods A cohort of 315 children with asthma, 6–11 years of age, was recruited for longitudinal follow-up in Fresno, California (USA). During the baseline visit, children were administered a respiratory symptom questionnaire and allergen skin-prick test. Three times a year, participants completed 14-day panels during which they answered symptom questions twice daily. Ambient air quality data from a central monitoring station were used to assign exposures to the following pollutants: particulate matter ≤ 2.5 μm in aerodynamic diameter, particulate matter between 2.5 and 10 μm in aerodynamic diameter (PM10–2.5), elemental carbon, nitrogen dioxide (NO2), nitrate, and O3. Results For the group as a whole, wheeze was significantly associated with short-term exposures to NO2 [odds ratio (OR) = 1.10 for 8.7-ppb increase; 95% confidence interval (CI), 1.02–1.20] and PM10–2.5 (OR = 1.11 for 14.7-μg/m3 increase; 95% CI, 1.01–1.22). The association with wheeze was stronger for these two pollutants in children who were skin-test positive to cat or common fungi and in boys with mild intermittent asthma. Conclusion A pollutant associated with traffic emissions, NO2, and a pollutant with bioactive constituents, PM10–2.5, were associated with increased risk of wheeze in asthmatic children living in Fresno, California. Children with atopy to cat or common fungi and boys with mild intermittent asthma were the subgroups for which we observed the largest associations.

Ambient temperature predicts sex ratios and male longevity

The theory that natural selection has conserved mechanisms by which women subjected to environmental stressors abort frail male fetuses implies that climate change may affect sex ratio at birth and male longevity. Using time series methods, we find that cold ambient temperatures during gestation predict lower secondary sex ratios and longer life span of males in annual birth cohorts composed of Danes, Finns, Norwegians, and Swedes born between 1878 (earliest year with complete life tables) and 1914 (last birth cohort for which male life span can be estimated). We conclude that ambient temperature affects the characteristics of human populations by influencing who survives gestation, a heretofore unrecognized effect of climate on humanity.

Perinatal morbidity associated with late preterm deliveries compared with deliveries between 37 and 40 weeks of gestation

Please cite this paper as: Cheng Y, Kaimal A, Bruckner T, Hallaron D, Caughey A. Perinatal morbidity associated with late preterm deliveries compared with deliveries between 37 and 40 weeks of gestation. BJOG 2011;118:1446–1454.

Increased neonatal mortality among normal-weight births beyond 41 weeks of gestation in California

OBJECTIVE The purpose of this study was to examine whether postterm gestational age increases the risk of neonatal mortality. STUDY DESIGN We retrieved data from 1,815,811 liveborn infants in California from 1999 to 2003. We excluded multiple births and congenital anomalies, as well as infants with a gestational age of less than 38 w0d, or greater than 42 w6d, weeks. We used multivariable logistic regression models to adjust for demographic variables thought to confound the association. RESULTS Compared to infants born at 38, 39, or 40 weeks, those born at 41 w0d to 42 w6d have a greater odds of neonatal mortality (aOR: 1.34, 95% CI, 1.08-1.65). Subdividing by gestational week, infants delivered at 41 w0d to 41 w6d showed elevated mortality relative to earlier term births (aOR: 1.37, 95% CI, 1.08-1.73). Additional analyses support this increased neonatal mortality across all normal birthweight categories. CONCLUSION Infants born beyond 41 w0d of gestation experience greater neonatal mortality relative to term infants born between 38 w0d and 40 w6d.

Selection in utero: a biological response to mass layoffs.

Most research describing the biological response to unemployment appears appropriately motivated by clinical or public health concerns and focuses on death, disease, and medical care. We argue that expanding the work to include other outcomes could contribute to basic science. As an example, we use the response to mass layoffs to discriminate between two explanations of low ratios of male to female live births in stressed populations. One explanation asserts that ambient stressors reduce the ratio of males to females conceived. The other argues that the maternal stress response selects against males in utero. We show that selection in utero better explains the observed data. We conclude that human adaptation to the economic environment deserves scrutiny from a wider array of scientists than it now receives. Am. J. Hum. Biol., 2010.

A sex-specific test of selection in utero

We find support for the hypothesis that changes in the monthly odds of a twin among live-born males predict subsequent and opposite changes in the odds of a twin among live-born females. The hypothesis arises from the long standing argument that natural selection has conserved mechanisms by which pregnant women in stressed populations spontaneously abort fetuses least likely to yield grandchildren. Previous attempts to empirically test this argument focus almost entirely on males. We contribute to the literature by showing that, consistent with the logic of natural selection, maternal adaptations to environmental changes likely have effects on the survival of both male and female conceptuses and fetuses.

Gender-specific selection in utero among contemporary human birth cohorts

Much literature argues that natural selection has conserved mechanisms by which stressed females cull frail males in utero. This argument implies that males from low sex ratio birth cohorts should, on average, live longer than those from high sex ratio cohorts. Research reports such associations but these tests use completed lifespan as the outcome and, therefore, must end with cohorts born in 1913 because too many males survive from more contemporary cohorts to determine average lifespan. The empirical literature does not, therefore, address whether selection affects male mortality in contemporary cohorts. We apply time-series methods to monthly cohorts born in California between 1989 and 2003 to measure the association between the ratio of male to female live births and infant mortality, controlling for all forms of autocorrelation that induce spurious correlations. Consistent with theories of selection in utero, we find a positive correlation between cohort sex ratio and male infant mortality. The results suggest that natural selection conserved the stress mechanism in females to end the gestation of relatively less fit males and that this mechanism manifests itself in contemporary human societies.