Why Is Infant Mortality Higher in Boys Than in Girls? A New Hypothesis Based on Preconception Environment and Evidence From a Large Sample of Twins

Infant mortality is higher in boys than girls in most parts of the world. This has been explained by sex differences in genetic and biological makeup, with boys being biologically weaker and more susceptible to diseases and premature death. At the same time, recent studies have found that numerous preconception or prenatal environmental factors affect the probability of a baby being conceived male or female. I propose that these environmental factors also explain sex differences in mortality. I contribute a new methodology of distinguishing between child biology and preconception environment by comparing male-female differences in mortality across opposite-sex twins, same-sex twins, and all twins. Using a large sample of twins from sub-Saharan Africa, I find that both preconception environment and child biology increase the mortality of male infants, but the effect of biology is substantially smaller than the literature suggests. I also estimate the interacting effects of biology with some intrauterine and external environmental factors, including birth order within a twin pair, social status, and climate. I find that a twin is more likely to be male if he is the firstborn, born to an educated mother, or born in certain climatic conditions. Male firstborns are more likely to survive than female firstborns, but only during the neonatal period. Finally, mortality is not affected by the interactions between biology and climate or between biology and social status.     

A Cross-cultural Comparison of the Dimensions of Child and Adolescent Life Satisfaction Reports

The factorial invariance of theMultidimensional Students' Life SatisfactionScale (MSLSS: Huebner, 1994) across twodivergent cultures (collectivistic vs.individualistic) was investigated with 835Korean and 822 US students in elementary,middle, and high schools. Psychometricproperties of the Korean version of the MSLSSwere acceptable. A series ofmultisample-confirmatory factor analyses (MCFA)demonstrated the cross-cultural similarity ofthe five-factor model for students' lifesatisfaction. The similar structure of MSLSSacross cultural groups provides the basis formeaningful international comparisons of lifesatisfaction among children and adolescents. Further cross-cultural research includingvarious culturally sensitive life satisfactiondomains may enhance understanding of culturaldifferences in the structure, correlates, andconsequences related to students' lifesatisfaction.     

Sleep hygiene practices and sleep duration in rotating-shift shiftworkers

Alcohol, caffeine and tobacco intake, exercise, activities on going to bed, and sleep-enhancing measures were assessed in 72 rotating-shift shiftworkers to determine whether these variables differ following day shifts and night-shifts, and which are related to sleep duration. Only alcohol intake and exercise prior to sleep, and feeling drowsy when retiring, differed following the night-shift. Workers generally followed recommendations concerning alcohol intake, sleeping immediately on retiring, and exercising, but acted contrary to recommendations concerning caffeine and nicotine intake. The only behaviour clearly associated with poor sleep was smoking, which reduced sleep duration.     

Closed Sequential and Multistage Inference on Binary Responses With or Without Replacement

Logistic regression is used by practitioners and researchers in many fields, but is undoubtedly used most frequently in medical and biostatistical applications. Maximum likelihood is generally the estimation method of choice, but we show that maximum likelihood can produce very poor results under certain conditions. Specifically, the poor performance of maximum likelihood in the case of rare events is known and we review research on this topic. We primarily examine the performance of maximum likelihood in the presence of near separation, which has apparently not been studied. Exact logistic regression is the logical alternative to maximum likelihood. We offer a comparison of the two methods of estimation.