When not to have another baby: An evolutionary approach to low fertility

Cultural niche construction with application to fertility control: A model for education and social transmission of contraceptive use

The evolution of a cultural trait may be affected by niche construction, or changes in the selective environment of that trait due to the inheritance of other cultural traits that make up a cultural background. This study investigates the evolution of a cultural trait, such as the acceptance of the idea of contraception, that is both vertically and horizontally transmitted within a homogeneous social network. Individuals may conform to the norm, and adopters of the trait have fewer progeny than others. In addition, adoption of this trait is affected by a vertically transmitted aspect of the cultural background, such as the preference for high or low levels of education. Our model shows that such cultural niche construction can facilitate the spread of traits with low Darwinian fitness while providing an environment that counteracts conformity to norms. In addition, niche construction can facilitate the 'demographic transition' by making reduced fertility socially accepted.

Fertility Dynamics and Life History Tactics Vary by Socioeconomic Position in a Transitioning Cohort of Postreproductive Chilean Women

Globally, mortality and fertility rates generally fall as resource abundance increases. This pattern represents an evolutionary paradox insofar as resource-rich ecological contexts can support higher numbers of offspring, a component of biological fitness. This paradox has not been resolved, in part because the relationships between fertility, life history strategies, reproductive behavior, and socioeconomic conditions are complex and cultural-historically contingent. We aim to understand how we might make sense of this paradox in the specific context of late-twentieth-century, mid–demographic transition Chile. We use distribution-specific generalized linear models to analyze associations between fertility-related life-history traits—number of offspring, ages at first and last reproduction, average interbirth interval, and average number of live births per reproductive span year—and socioeconomic position (SEP) using data from a cohort of 6,802 Chilean women born between 1961 and 1970. We show that Chilean women of higher SEP have shorter average interbirth intervals, more births per reproductive span year, later age at first reproduction, earlier ages at last reproduction, and, ultimately, fewer children than women of lower SEP. Chilean women of higher SEP consolidate childbearing over a relatively short time span in the middle of their reproductive careers, whereas women of lower SEP tend to reproduce over the entirety of their reproductive lifespans. These patterns may indicate that different SEP groups follow different pathways toward declining fertility during the demographic transition, reflecting different life-history trade-offs in the process.

Do Criminals Live Faster Than Soldiers and Firefighters? : A Comparison of Biodemographic and Psychosocial Dimensions of Life History Theory

A high risk of morbidity-mortality caused by a harsh and unpredictable environment is considered to be associated with a fast life history (LH) strategy, commonly linked with criminal behavior. However, offenders are not the only group with a high exposure to extrinsic morbidity-mortality. In the present study, we investigated the LH strategies employed by two groups of Polish men: incarcerated offenders (N = 84) as well as soldiers and firefighters (N = 117), whose professions involve an elevated risk of injury and premature death. The subjects were asked to complete the Mini-K (used as a psychosocial LH indicator) and a questionnaire which included a number of biodemographic LH variables. Although biodemographic and psychosocial LH indicators should be closely linked with each other, the actual connection between them is unclear. Thus, this study was driven by two aims: comparing LH strategies in two groups of men with a high risk of premature morbidity-mortality and investigating the relationship between the biodemographic and psychosocial LH dimensions. The study showed that incarcerated men employed faster LH strategies than soldiers and firefighters, but only in relation to biodemographic variables (e.g., number of siblings, age of sexual initiation, life expectancy). No intergroup differences emerged regarding psychosocial LH indicators. Moreover, the correlation analysis showed a weak association between biodemographic and psychosocial LH indicators. The results strengthen the legitimacy of incorporating biodemographic LH traits into research models and indicate the need for further research on the accuracy of the Mini-K. The possible explanations for the intergroup differences in LH strategies are discussed.

The life course cube: A tool for studying lives

This paper proposes a conceptualization of the life course as a set of behavioral processes characterized by interdependencies that cross time, life domains, and levels of analysis. We first discuss the need for a systematized approach to life course theory that integrates parallel and partially redundant concepts developed in a variety of disciplines. We then introduce the 'life course cube,' which graphically defines and illustrates time-domain-level interdependencies and their multiple interactions that are central to understanding life courses. Finally, in an appendix, we offer a formal account of these interactions in a language that can be readily adopted across disciplines. Our aim is to provide a consistent and parsimonious foundation to further develop life course theories and methods and integrate life course scholarship across disciplines.

Fertility Intentions and Residential Relocations

This research addresses the question of whether fertility intentions (before conception) are associated with residential relocations and the distance of the relocation. We empirically tested this using data from two birth cohorts (aged 24-28 and 34-38 in the first survey wave) of the German Family Panel (pairfam) and event history analysis. Bivariate analyses showed that coupled individuals relocated at a higher rate if they intended to have a(nother) child. We found substantial heterogeneity according to individuals' age and parental status, particularly for outside-town relocations. Childless individuals of average age at family formation-a highly mobile group-relocated at a lower rate if they intended to have a child. In contrast, older individuals who already had children-the least-mobile group-relocated at a higher rate if they intended to have another child. Multivariate analyses show that these associations are largely due to adjustments in housing and other living conditions. Our results suggest that anticipatory relocations (before conception) to adapt to growing household size are importantly nuanced by the opportunities and rationales of couples to adjust their living conditions over the life course. Our research contributes to the understanding of residential mobility as a by-product of fertility decisions and, more broadly, evidences that intentions matter and need to be considered in the analysis of family life courses.

Give one species the task to come up with a theory that spans them all: what good can come out of that?

Does the progress in understanding evolutionary theory depend on the species that is doing the investigation? This question is difficult to answer scientifically, as we are dealing with an n = 1 scenario: every individual who has ever written about evolution is a human being. I will discuss, first, whether we get the correct answer to questions if we begin with ourselves and expand outwards, and second, whether we might fail to ask all the interesting questions unless we combat our tendencies to favour taxa that are close to us. As a whole, the human tendency to understand general biological phenomena via 'putting oneself in another organism's shoes' has upsides and downsides. As an upside, our intuitive ability to rethink strategies if the situation changes can lead to ready generation of adaptive hypotheses. Downsides occur if we trust this intuition too much, and particular danger zones exist for traits where humans are an unusual species. I argue that the levels of selection debate might have proceeded differently if human cooperation patterns were not so unique, as this brings about unique challenges in biology teaching; and that theoretical insights regarding inbreeding avoidance versus tolerance could have spread faster if we were not extrapolating our emotional reactions to incest disproportionately depending on whether we study animals or plants. I also discuss patterns such as taxonomic chauvinism, i.e. less attention being paid to species that differ more from human-like life histories. Textbooks on evolution reinforce such biases insofar as they present, as a default case, systems that resemble ours in terms of life cycles and other features (e.g. gonochorism). Additionally, societal norms may have led to incorrect null hypotheses such as females not mating multiply.

Support for new mothers and fertility in the United Kingdom: Not all support is equal in the decision to have a second child

Low fertility across Europe highlights the need to understand reproductive decisions in high-income countries better. Availability of support may be one factor influencing reproductive decisions, though within high-income countries availability varies between environments, including socio-economic environments. We test whether receiving higher levels of support, from different sources (informal and formal) and of different types (practical and emotional), is positively correlated with second births in the United Kingdom (UK) Millennium Cohort Study, and whether these relationships differ by socio-economic position (SEP). Our hypothesis is only partially supported: receiving emotional support correlates with higher likelihood of second birth, but the opposite is true for practical support. Availability of different types of support varies across SEP, but relationships between support and fertility are similar, with one exception: kin-provided childcare increases the likelihood of birth only among lower-SEP women. Our results highlight that not all support is equal in the decision to have a second child.

Resource Transfers and Human Life-History Evolution

Formal models of life-history evolution have been used to illuminate both the peculiarities of the human life cycle and its commonalities with those of other taxa. Understanding reproductive decisions in both the contemporary market-based economies of wealthy nation-states and rapidly changing populations largely of the Global South presents particular challenges to evolutionary life-history theory for several reasons. These include (a) the rapidity with which reproductive patterns change, (b) the magnitude of fertility reduction from previous equilibria, and (c) the frequent absence or even reversal of expected wealth-fertility gradients. These empirical challenges have been met to an increasing extent by specifically incorporating durable wealth and resource transfers into more traditional life-history models. Such relatively new models build on classical life-history theory to generate novel predictions. Among these are quite robust predictions that the existence of heritable wealth will decrease optimal fertility and that, once the system of resource transfers is established, fertility and resource transfers will coevolve.

A panel data analysis of the probability of childbirth in a Japanese sample: New evidence of the two-child norm

OBJECTIVES

To reveal the conditions that could facilitate childbirth in modern humans, it is necessary to analyze not only cross-sectional surveys but also panel data that track the same person for a long period. In this study, we analyzed factors that would influence the probability of childbirth.

METHODS

We analyzed Japanese panel data with a Cox proportional hazard model. Subjects of our analysis were married women and their childbirth records from 2004 to 2009.

RESULTS

Contrary to the predictions based on the theory of behavioral ecology, we found no positive relationship between good parental conditions for childcare, such as high income, increase in income, or coresidence with parents (i.e., grandparents of children), and the occurrence of childbirth. We found that the number of existing children had a significant impact on the probability of childbirth. The likelihood of further childbirth by couples with one child was nearly equal to that of childless ones. However, the corresponding likelihood of couples with two children was about five times lower than that of childless ones.

CONCLUSIONS

The total fertility rates in modern developed societies are quite low and couples prefer having two children. This trend is known as the two-child norm, but it is a paradoxical phenomenon in terms of fitness maximization. Our result provided new quantitative evidence of this norm. This study revealed that the number of existing children being less than two was one of the factors associated with further childbearing in our Japanese sample.

With the help of kin? Household composition and reproduction in the Netherlands, 1842-1920

Relatives play an important role in human reproduction according to evolutionary theories of reproductive behavior, but previous empirical studies show large differences in the effects of kin on fertility outcomes. In our paper we examine the effect of co-resident kin and non-kin on the length of birth intervals over the reproductive life course of Dutch women born between 1842 and 1920. We estimate Cox proportional hazard models for parity progression based on the presence of kin and non-kin in the household while controlling for a large number of individual and community-level characteristics. We find that couples living with their brothers experienced shorter birth intervals whereas couples residing with a widowed father had relatively longer birth intervals. The effects of these types of kin on reproduction were most pronounced up to the birth of the fifth child, but not thereafter. We found no effect for mothers or other types of kin.