Generational
Anna Goodman, Ilona Koupil & David Lawson
Proceedings of the Royal Society: Biological Sciences, forthcoming
Abstract:
Adaptive accounts of modern low human fertility argue that small family size maximizes the inheritance of socioeconomic resources across generations and may consequently increase long-term fitness. This study explores the long-term impacts of fertility and socioeconomic position (SEP) on multiple dimensions of descendant success in a unique Swedish cohort of 14 000 individuals born during 1915-1929. We show that low fertility and high SEP predict increased descendant socioeconomic success across four generations. Furthermore, these effects are multiplicative, with the greatest benefits of low fertility observed when SEP is high. Low fertility and high SEP do not, however, predict increased descendant reproductive success. Our results are therefore consistent with the idea that modern fertility limitation represents a strategic response to the local costs of rearing socioeconomically competitive offspring, but contradict adaptive models suggesting that it maximizes long-term fitness. This indicates a conflict in modern societies between behaviours promoting socioeconomic versus biological success. This study also makes a methodological contribution, demonstrating that the number of offspring strongly predicts long-term fitness and thereby validating use of fertility data to estimate current selective pressures in modern populations. Finally, our findings highlight that differences in fertility and SEP can have important long-term effects on the persistence of social inequalities across generations.
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The Black-White Gap in Non-Marital Fertility: Education and Mates in Segmented Marriage Markets
Joe Stone
Journal of Labor Research, September 2012, Pages 328-336
Abstract:
This study is the first to find that mate availability explains much of the race gap in non-marital fertility in the United States. Both a general and an education-based metric have strong effects. The novel statistical power arises from difference-in-differences for blacks and whites, multiple cohorts, periods, and coefficient restrictions consistent with both the data and models in which differences in mate availability can induce blacks and whites to respond in opposite directions to changes in mate availability. Results are robust to several alternative specifications and tests and appear relevant where marriages are segmented along racial, religious, or other lines.
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Back to the Future? Abortion Before & After Roe
Theodore Joyce, Ruoding Tan & Yuxiu Zhang
NBER Working Paper, August 2012
Abstract:
Next year marks the 40th anniversary of the U.S. Supreme Court decision in Roe v. Wade. We use unique data on abortions performed in New York State from 1971-1975 to analyze the impact of legalized abortion in New York on abortion and birth rates of non-residents. We estimate that abortion rates declined by 12.0 percent for every hundred miles a woman lived from New York in the years before Roe. If Roe were overturned average travel distance to the nearest abortion provider would increase by 157 miles in the 31 states expected to prohibit abortion. Under this scenario abortion rates would fall by 14.9 percent nationally, resulting in at most, 178,800 additional births or 4.2 percent of the U.S. total in 2008. A ban in 17 states would result in a 6.0 percent decline in abortions and at most, 1.7 percent rise in births.
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Joshua Rolnick & John Vorhies
Journal of Public Health Policy, August 2012, Pages 348-362
Abstract:
Although US federal law requires all American states to permit abortion within their borders, states retain authority to impose restrictions. We used hospital discharge data to study the rates of major abortion complications in 23 states from 2001 to 2008 and their relationship to two laws: (i) restrictions on Medicaid - the state insurance programs for the poor - funding, and (ii) mandatory delays before abortion. Of 131 000 000 discharges in the data set, 10 980 involved an abortion complication. The national rate for complications was 1.90 per 1000 abortions (95 per cent CI: 1.57-2.23). Eleven states required mandatory delays and 12 restricted funding for Medicaid participants. After controlling for socio-economic characteristics and the pregnancy complication rate, legal restrictions were associated with lower complication rates: mandatory delays (OR 0.79 (0.65-0.95)) and restricted Medicaid funding (OR 0.74 (0.61-0.90)). This result may reflect the fact that states without restrictions perform a higher percentage of second-trimester abortions. This study is the first to assess the association between legal restrictions on abortion and complication rates.
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Hypotheses on the stability and variation of human sex ratios at birth
William James
Journal of Theoretical Biology, 7 October 2012, Pages 183-186
Abstract:
Human sex ratios at birth simultaneously show both significant variation with a number of variables, and striking stability across time. Hypotheses on these features are discussed here.
A) The causes of the stability are not established.
B) There are several hypotheses which purport to explain sex ratio variation.
1. The Trivers-Willard hypothesis has had only limited success. This may be because (from a methodological standpoint) it has an unusual provenance in that it is not a response to a perceived need for explanation of an observed phenomenon. At present there seems too much evidence in its favour for this hypothesis to be rejected, and too much against it, for it to be accepted.
2. My hypothesis proposes that hormone concentrations (of both parents) around the time of conception partially control the sex of the zygote. A substantial quantity of data has been adduced in favour of this hypothesis. But it cannot explain all types of variation of sex ratios at birth.
3. It has been proposed by Catalano that other variation in sex ratios at birth is associated with maternal stress during pregnancy. He and his co-workers have adduced substantial quantities of data to support this hypothesis too.
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Season of Birth and Later Outcomes: Old Questions, New Answers
Kasey Buckles & Daniel Hungerman
Review of Economics and Statistics, forthcoming
Abstract:
Season of birth is associated with later outcomes; what drives this association remains unclear. We consider a new explanation: variation in maternal characteristics. We document large changes in maternal characteristics for births throughout the year; winter births are disproportionally realized by teenagers and the unmarried. Family background controls explain nearly half of season-of-birth's relation to adult outcomes. Seasonality in maternal characteristics is driven by women trying to conceive; we find no seasonality among unwanted births. Prior seasonality-in-fertility research focuses on conditions at conception; here expected conditions at birth drive variation in maternal characteristics while conditions at conception are unimportant.
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Declining fertility and economic well-being: Do education and health ride to the rescue?
Klaus Prettner, David Bloom & Holger Strulik
Labour Economics, forthcoming
Abstract:
It is widely argued that declining fertility slows the pace of economic growth in industrialized countries through its negative effect on labor supply. There are, however, theoretical arguments suggesting that the effect of falling fertility on effective labor supply can be offset by associated behavioral changes. We formalize these arguments by setting forth a dynamic consumer optimization model that incorporates endogenous fertility as well as endogenous education and health investments. The model shows that a fertility decline induces higher education and health investments that are able to compensate for declining fertility under certain circumstances. We assess the theoretical implications by investigating panel data for 118 countries over the period 1980 to 2005 and show that behavioral changes partly mitigate the negative impact of declining fertility on effective labor supply.
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Has Democratization Reduced Infant Mortality in Sub-Saharan Africa? Evidence from Micro Data
Masayuki Kudamatsu
Journal of the European Economic Association, forthcoming
Abstract:
Does democracy help babies survive in sub-Saharan Africa? By using retrospective fertility surveys conducted in 28 African countries, I compare the survival of infants born to the same mother before and after democratization to disentangle the effect of democracy from that of changes in population characteristics, which is infeasible with country-level statistics on infant mortality. I find that infant mortality falls by 1.2 percentage points, 12% of the sample mean, after democratization in the post-Cold War period. Relevant aspects of democracy appear to be the combination of multiparty elections and leadership change.
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Psychological stress and reproductive aging among pre-menopausal women
M.E. Bleil et al.
Human Reproduction, September 2012, Pages 2720-2728
Background: Life history models suggest that biological preparation for current versus longer term reproduction is favored in environments of adversity. In this context, we present a model of reproductive aging in which environmental adversity is proposed to increase the number of growing follicles at the cost of hastening the depletion of the ovarian reserve over time. We evaluated this model by examining psychological stress in relation to reproductive aging indexed by antral follicle count (AFC), a marker of total ovarian reserve. We hypothesized that stress would be related to (i) higher AFC in younger women, reflecting greater reproductive readiness as well as (ii) greater AFC loss across women, reflecting more accelerated reproductive aging.
Methods: In a multi-ethnic, community sample of 979 participants [ages 25-45 (mean (standard deviation) = 35.2 (5.5)); 27.5% Caucasian] in the Ovarian Aging study, an investigation of the correlates of reproductive aging, the interaction of age-x-stress was assessed in relation to AFC to determine whether AFC and AFC loss varied across women experiencing differing levels of stress. Stress was assessed by the perceived stress scale and AFC was assessed by summing the total number of antral follicles visible by transvaginal ultrasound.
Results: In linear regression examining AFC as the dependent variable, covariates (race/ethnicity, socio-economic status, menarcheal age, hormone-containing medication for birth control, parity, cigarette smoking, bodymass index, waist-to-hip ratio) and age were entered on step 1, stress on step 2 and the interaction term (age-x-stress) on step 3. On step 3, significant main effects showed that older age was related to lower AFC (b = -0.882, P = 0.000) and greater stress was related to higher AFC (b = 0.545, P = 0.005). Follow-up analyses showed that the main effect of stress on AFC was present in the younger women only. A significant interaction term (b = -0.036, P = 0.031) showed the relationship between age and AFC varied as function of stress. When the sample was divided into tertiles of stress, the average follicle loss was -0.781, -0.842 and -0.994 follicles/year in the low-, mid- and high-stress groups, respectively.
Conclusions: Psychological stress was related to higher AFC among younger women and greater AFC decline across women, suggesting that greater stress may enhance reproductive readiness in the short term at the cost of accelerating reproductive aging in the long term. Findings are preliminary, however, due to the cross-sectional nature of the current study.
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Grazyna Jasienska, Michal Jasienski & Peter Ellison
Evolution and Human Behavior, forthcoming
Abstract:
Most research shows that fatherhood is related to reduced testosterone (T) levels, but relationships between the number of children and T levels are not addressed. In humans, paternal care usually involves obtaining adequate resources to support children, which may require engaging in male-male competition and maintaining high T levels. We hypothesize that T levels in fathers should increase with increasing family size. In 78 Polish men, aged 30 to 77 years, the number of children was significantly correlated with paternal T levels, but the direction of this relationship was dependent on the fathers' education. In agreement with our hypothesis, in men with below-college education, T levels increased with increasing number of children. In contrast, in men with college education, the number of children was negatively related to paternal T levels. Drop in T levels throughout the day tended to be less pronounced the more children fathers had, irrespective of their educational level. Our results suggest that a hypothesis of simple trade-offs between mating and parenting effort may be too simplistic to explain changes in testosterone response to parenting in human males. In order to understand functional response of changes in T levels, it is crucial to account for family size and socioeconomic factors. However, due to the cross-sectional study design, we cannot exclude the possibility that T levels influenced reproductive behavior (rather than vice versa) and thus the number of children produced by men.
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Voice and Handgrip Strength Predict Reproductive Success in a Group of Indigenous African Females
Jeremy Atkinson et al.
PLoS ONE, August 2012
Abstract:
Evolutionary accounts of human traits are often based on proxies for genetic fitness (e.g., number of sex partners, facial attractiveness). Instead of using proxies, actual differences in reproductive success is a more direct measure of Darwinian fitness. Certain voice acoustics such as fundamental frequency and measures of health such as handgrip strength correlate with proxies of fitness, yet there are few studies showing the relation of these traits to reproduction. Here, we explore whether the fundamental frequency of the voice and handgrip strength account for differences in actual reproduction among a population of natural fertility humans. Our results show that both fundamental frequency and handgrip strength predict several measures of reproductive success among a group of indigenous Namibian females, particularly amongst the elderly, with weight also predicting reproductive outcomes among males. These findings demonstrate that both hormonally regulated and phenotypic quality markers can be used as measures of Darwinian fitness among humans living under conditions that resemble the evolutionary environment of Homo sapiens. We also argue that these findings provide support for the Grandmother Hypothesis.
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Maroussia Favre & Didier Sornette
Journal of Theoretical Biology, 7 October 2012, Pages 43-54
Abstract:
The Time to the Most Recent Common Ancestor (TMRCA) based on human mitochondrial DNA (mtDNA) is estimated to be twice that based on the non-recombining part of the Y chromosome (NRY). These TMRCAs have special demographic implications because mtDNA is transmitted only from mother to child, while NRY is passed along from father to son. Therefore, the former locus reflects female history, and the latter, male history. To investigate what caused the two-to-one female-male TMRCA ratio rF/M=TF/TM in humans, we develop a forward-looking agent-based model (ABM) with overlapping generations. Our ABM simulates agents with individual life cycles, including life events such as reaching maturity or menopause. We implemented two main mating systems: polygynandry and polygyny with different degrees in between. In each mating system, the male population can be either homogeneous or heterogeneous. In the latter case, some males are ‘alphas' and others are ‘betas', which reflects the extent to which they are favored by female mates. A heterogeneous male population implies a competition among males with the purpose of signaling as alpha males. The introduction of a heterogeneous male population is found to reduce by a factor 2 the probability of finding equal female and male TMRCAs and shifts the distribution of rF/M to higher values. In order to account for the empirical observation of the factor 2, a high level of heterogeneity in the male population is needed: less than half the males can be alphas and betas can have at most half the fitness of alphas for the TMRCA ratio to depart significantly from 1. In addition, we find that, in the modes that maximize the probability of having 1.5
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Adolescent fertility and risky environments: A population-level perspective across the lifespan
Caitlyn Placek & Robert Quinlan
Proceedings of the Royal Society: Biological Sciences, 7 October 2012, Pages 4003-4008
Abstract:
Timing of first reproduction is a key life-history variable with important implications for global economic development and health. Life-history theory predicts that human reproductive strategies are shaped by mortality regimes. This study provides the first test of the relationship between population-level adolescent fertility (AF) and extrinsic risk at two time points. Data are from United Nations database and were analysed using mediation and moderation techniques. The goals were to determine whether (i) early risk has a stronger impact on fertility than current risk; (ii) current risk mediates the relationship between early risk and fertility outcomes; and (iii) different levels of early risk influence the relationship between current risk and fertility. Results indicated that current risk partially mediated the relationship between early risk and fertility, with early risk having the strongest impact on reproduction. Measures for early and current mortality did not show significant interaction effects. However, a series of separate regression analyses using a quantile split of early risk indicated that high levels of early risk strengthened the relationship between current risk and AF. Overall, these findings demonstrate that reproductive strategies are significantly influenced by fluctuations of early mortality as well as current environmental cues of harshness.
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Rate of de novo mutations and the importance of father's age to disease risk
Augustine Kong et al.
Nature, 23 August 2012, Pages 471-475
Abstract:
Mutations generate sequence diversity and provide a substrate for selection. The rate of de novo mutations is therefore of major importance to evolution. Here we conduct a study of genome-wide mutation rates by sequencing the entire genomes of 78 Icelandic parent-offspring trios at high coverage. We show that in our samples, with an average father's age of 29.7, the average de novo mutation rate is 1.20 × 10-8 per nucleotide per generation. Most notably, the diversity in mutation rate of single nucleotide polymorphisms is dominated by the age of the father at conception of the child. The effect is an increase of about two mutations per year. An exponential model estimates paternal mutations doubling every 16.5 years. After accounting for random Poisson variation, father's age is estimated to explain nearly all of the remaining variation in the de novo mutation counts. These observations shed light on the importance of the father's age on the risk of diseases such as schizophrenia and autism.
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Metabolic hypothesis for human altriciality
Holly Dunsworth et al.
Proceedings of the National Academy of Sciences, forthcoming
Abstract:
The classic anthropological hypothesis known as the "obstetrical dilemma" is a well-known explanation for human altriciality, a condition that has significant implications for human social and behavioral evolution. The hypothesis holds that antagonistic selection for a large neonatal brain and a narrow, bipedal-adapted birth canal poses a problem for childbirth; the hominin "solution" is to truncate gestation, resulting in an altricial neonate. This explanation for human altriciality based on pelvic constraints persists despite data linking human life history to that of other species. Here, we present evidence that challenges the importance of pelvic morphology and mechanics in the evolution of human gestation and altriciality. Instead, our analyses suggest that limits to maternal metabolism are the primary constraints on human gestation length and fetal growth. Although pelvic remodeling and encephalization during hominin evolution contributed to the present parturitional difficulty, there is little evidence that pelvic constraints have altered the timing of birth.
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African-American/white differences in the age of menarche: Accounting for the difference
Patricia Reagan et al.
Social Science & Medicine, October 2012, Pages 1263-1270
Abstract:
Lifetime health disparity between African-American and white females begins with lower birthweight and higher rates of childhood overweight. In adolescence, African-American girls experience earlier menarche. Understanding the origins of these health disparities is a national priority. There is growing literature suggesting that the life course health development model is a useful framework for studying disparities. The purpose of this study was to quantify the influence of explanatory factors from key developmental stages on the age of menarche and to determine how much of the overall race difference in age of menarche they could explain. The factors were maternal age of menarche, birthweight, poverty during early childhood (age 0 through 5 years), and child BMI z-scores at 6 years. The sample, drawn from the US National Longitudinal Surveys of Youth Child-Mother file, consisted of 2337 girls born between 1978 and 1998. Mean age of menarche in months was 144 for African-American girls and 150 for whites. An instrumental variable approach was used to estimate a causal effect of child BMI z-score on age of menarche. The instrumental variables were pre-pregnancy BMI, high gestational weight gain and smoking during pregnancy. We found strong effects of maternal age of menarche, birthweight, and child BMI z-score (-5.23, 95% CI [-7.35,-3.12]) for both African-Americans and whites. Age of menarche declined with increases in exposure to poverty during early childhood for whites. There was no effect of poverty for African-Americans. We used Oaxaca decomposition techniques to determine how much of the overall race difference in age of menarche was attributable to race differences in observable factors and how much was due to race dependent responses. The African-American/white difference in childhood BMI explained about 18% of the overall difference in age of menarche and birthweight differences explained another 11%.
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Wendie Robbins et al.
Biology of Reproduction, forthcoming
Purpose: We tested the hypothesis that 75 gm of whole-shelled walnuts/day added to a Western-style diet of healthy young men would beneficially affect semen quality.
Methods: A randomized, parallel two-group, dietary intervention trial with single-blind masking of outcome assessors, was conducted with 117 healthy men, age 21-35 years, who routinely consumed a Western-style diet. Primary outcome evaluated was improvement from baseline to 12 weeks in conventional semen parameters and sperm aneuploidy. Secondary endpoints included blood serum and sperm fatty acid (FA) profiles, sex hormones, and serum folate.
Conclusions: The group consuming walnuts (n=59) experienced improvement in sperm vitality, motility, and morphology and the group continuing their usual diet but avoiding tree nuts (n=58) saw no change. Comparing differences from baseline between the groups, significance was found for vitality p=0.003, motility p=0.009, and morphology p=0.04. Serum FA profiles improved in the walnut group with increases in omega-6 (p=0.0004) and omega-3 (p=0.0007) but not the control group. Only the plant source of omega-3, α-linolenic acid (ALA), increased (p=0.0001). Sperm aneuploidy was inversely correlated with sperm ALA, particularly sex chromosome nullisomy (-0.41, p=0.002). Findings demonstrated that walnuts added to a Western-style diet improved sperm vitality, motility and morphology.
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Mitochondria, Maternal Inheritance, and Male Aging
Florencia Camus, David Clancy & Damian Dowling
Current Biology, forthcoming
Abstract:
The maternal transmission of mitochondrial genomes invokes a sex-specific selective sieve, whereby mutations in mitochondrial DNA can only respond to selection acting directly on females. In theory, this enables male-harming mutations to accumulate in mitochondrial genomes when these same mutations are neutral, beneficial, or only slightly deleterious in their effects on females. Ultimately, this evolutionary process could result in the evolution of male-specific mitochondrial mutation loads; an idea previously termed Mother's Curse. Here, we present evidence that the effects of this process are broader than hitherto realized, and that it has resulted in mutation loads affecting patterns of aging in male, but not female Drosophila melanogaster. Furthermore, our results indicate that the mitochondrial mutation loads affecting male aging generally comprise numerous mutations over multiple sites. Our findings thus suggest that males are subject to dramatic consequences that result from the maternal transmission of mitochondrial genomes. They implicate the diminutive mitochondrial genome as a hotspot for mutations that affect sex-specific patterns of aging, thus promoting the idea that a sex-specific selective sieve in mitochondrial genome evolution is a contributing factor to sexual dimorphism in aging, commonly observed across species.