Salivary steroid immunoassays are widely used in psychoneuroendocrinology to investigate the psychological effects of menstrual cycle phase. Though manufacturers advertise their assays as suitable, they have not been rigorously validated for this purpose. We collated data from eight studies across more than 1,200 women and more than 9,500 time points. All studies measured estradiol and progesterone and had at least one independent indicator of cycle phase (day in cycle relative to the luteinising hormone surge or a menstrual onset). Seven studies collected saliva; one study collected serum. In serum, all non-steroid cycle phase measures strongly predicted steroids in the expected manner. By contrast, salivary immunoassays of estradiol were only weakly predictable from cycle phase and showed an upward bias compared to expectations from serum. For salivary immunoassays of progesterone, predictability from cycle phase was more mixed, but two widely used assays performed poorly. Imputing average serum steroid levels from cycle phase may yield more valid values than several widely used salivary immunoassays. Tandem mass spectrometry may provide a valid alternative to widely used immunoassays and could be combined with imputation.

Few studies have examined birth order effects on personality in countries that are not Western, educated, industrialized, rich, and democratic (WEIRD). However, theories have generally suggested that interculturally universal family dynamics are the mechanism behind birth order effects, and prominent theories such as resource dilution would predict even stronger linear effects in poorer countries. Here, we examine a subset of up to 11 188 participants in the Indonesian Family Life Survey to investigate whether later-borns differ from earlier-borns in intelligence, educational attainment, Big Five, and risk aversion. Analyses were performed using within-family designs in mixed-effects models. In model comparisons, we tested for linear and non-linear birth order effects as well as for possible interactions of birth order and sibship size. Our estimated effect sizes are consistent with the emerging account of birth order as having relatively little impact on intelligence, Big Five, and risk aversion. We found a non-linear pattern for educational attainment that was not robust to imputation of missing data and not aligned with trends in WEIRD countries. Overall, the small birth order effects reported in other studies appear to be culturally specific.

No abstract

Few studies have examined birth order effects on personality in countries that are not Western, educated, industrialized, rich, and democratic (WEIRD), even though theories have generally suggested interculturally universal family dynamics as the mechanism behind birth order effects, and prominent theories such as resource dilution would even predict stronger effects in poorer countries. Here, we investigate a subset of up to 11,188 participants of the Indonesian Family Life Survey, an ongoing representative panel study, to study whether later-born siblings differ from earlier-borns in intelligence, educational attainment, personality, and risk aversion. Analyses were performed using within-family designs in mixed-effects models. In model comparisons we tested for linear and non-linear birth order effects as well as for possible interactions of birth order and sibship size. Our estimated effect sizes are consistent with the emerging account of birth order as having relatively little impact on intelligence, education, personality, and risk aversion; and they exclude recent estimates from WEIRD populations based on large sample sizes. Thus, even the small effects of birth order reported in other studies appear to be culturally specific.

Many of the women who take hormonal contraceptives discontinue because of unwanted side effects, including negative psychological effects. Yet scientific evidence of psychological effects is mixed, partly because causal claims are often based on correlational data. In correlational studies, possible causal effects can be difficult to separate from selection effects, attrition effects, and reverse causality. Contraceptive use and, according to the congruency hypothesis, congruent contraceptive use (whether a woman’s current use/non-use of a hormonal contraceptive is congruent with her use/non-use at the time of meeting her partner) have both been thought to influence relationship quality and sexual functioning. In order to address potential issues of observed and unobserved selection effects in correlational data, we studied a sample of up to 1,179 women to investigate potential effects of contraceptive use and congruent contraceptive use on several measures of relationship quality and sexual functioning: perceived partner attractiveness, relationship satisfaction, sexual satisfaction, and diary measurements including libido, frequency of vaginal intercourse, and frequency of masturbation. No evidence for substantial effects was found except for a positive effect of hormonal contraceptives on frequency of vaginal intercourse and a negative effect of hormonal contraceptives on frequency of masturbation. These effects were robust to the inclusion of observed confounders, and their sensitivity to unobserved confounders was estimated. No support for the congruency hypothesis was found. Our correlational study was able to disentangle, to some extent, causal effects of hormonal contraceptives from selection effects by estimating the sensitivity of reported effects. To reconcile experimental and observational evidence on hormonal contraceptives, future research should scrutinize the role of unobserved selection effects, attrition effects, and reverse causality.

This investigation was funded by a grant from the Carlsberg Foundation (CF16-0444) to the fifth author. We gratefully thank the NBBL/JBBL (Germany) and Christoph Coura in particular for their help in collecting the data. Note that the NBBL/JBBL had no influence on the research question, analyses, results, or the preparation of this manuscript at all.

Many of the women who take hormonal contraceptives discontinue because of unwanted side effects, including negative psychological effects. Yet scientific evidence of psychological effects is mixed and, partly because causal claims are often based on correlational data. In correlational studies, possible causal effects can be difficult to separate from selection effects, attrition effects, and reverse causality. Contraceptive use and, according to the congruency hypothesis, congruent contraceptive use (whether a woman’s current use/non-use of a hormonal contraceptive is congruent with her use/non-use at the time of meeting her partner) have both been thought to influence relationship quality and sexual functioning. In order to address potential issues of observed and unobserved selection effects in correlational data, we studied a sample of up to 1,179 women to investigate potential effects of contraceptive use and congruent contraceptive use on several measures of relationship quality and sexual functioning: perceived partner attractiveness, relationship satisfaction, sexual satisfaction, and diary measurements including libido, frequency of vaginal intercourse, and frequency of masturbation. No evidence for substantial effects was found except for a positive effect of hormonal contraceptives on frequency of vaginal intercourse and a negative effect of hormonal contraceptives on frequency of masturbation. These effects were robust to the inclusion of observed confounders, and their sensitivity to unobserved confounders was estimated. No support for the congruency hypothesis was found. Our correlational study was able to disentangle, to some extent, causal effects of hormonal contraceptives from selection effects by estimating the sensitivity of reported effects. To reconcile experimental and observational evidence on hormonal contraceptives, future research should scrutinize the role of unobserved selection effects, attrition effects, and reverse causality.