Why we need interdisciplinarity when studying fertility and health
This article is part of our "Perspectives" series
by Vegard Skirbekk
Centre for Fertility and Health, Norwegian Institute of Public Health and Columbia Aging Center at Columbia University
Scientists interested in health related aspects of fertility and changing family constellations are often highly specialized – much of the research carried out primarily focus on variables and models specific to certain disciplines with an aim of publication in disciplinary journals. Key reasons scientists do not integrate insights and mechanisms from other disciplines can be insufficient time, resources and competence, and a lack of the necessary motivation or incentives. In this statement of direction for our new research centre, I make the case for interdisciplinary research in the study of fertility and health. I provide a few examples, including how integration of a broader set approaches for measuring the transition to adulthood is crucial for understanding the heterogeneity in childbearing patterns, as well as how a changing selection into childlessness affect relations to health outcomes.
When studying fertility behaviours and health implications many researchers stress either biological, psychological, socioeconomic or cultural factors. However, fertility patterns have been shown to be influenced by a range of such determinants, and failure to account for these can likely lead to biased results. Fertility drivers include social factors affecting the timing of fertility (e.g., education, travels and self-realization, career establishment, partnering patterns); whether one can have children (such as polycystic ovary syndrome, semen quality, availability of assisted reproductive technologies); cultural factors (including social values, preferences for family formation versus other life goals); family influences (e.g., family members providing social meaning and practical support, belonging to social groups where childbearing is encouraged or discouraged); and genetic factors (affecting fecundity, risks behaviours, heritable disease incidence). These drivers should ideally be integrated in joint models. Having solely a disciplinary focus is likely to give less understanding of causal effects and mediating mechanisms, and would explain less of the variance.
A central motivation for the centre is how fertility patterns have changed in Norway and for other countries with similar socioeconomic developments. The population proportions without children are growing throughout the world (CDC 2018, Sobotka 2017, Waren and Pals 2013) – e.g., in both the US and Norway, the proportion of men who are childless towards the end of their reproductive years is now around one quarter, see Figures 1 and 2.
Fig 1) Norwegian probability of childlessness at age 45 over time. Source: (Statistics Norway 2017)
Fig 2) The share who are parents in the US by age for the 2006-2010 period. Source: (CDC 2018)
The selection of those who do not have children differs over time, and this has implications for the health of children and adults.
Since families are central for influencing lifestyles and health relevant behaviours through social and genetic effects, the family dimension can be central when studying disease etiology. Those who are childless today are different from those who were childless just a few decades ago, e.g., higher social status (income, rank-order within social hierarchy) has in several world regions shifted from being positively related to fertility to be neutrally or a negatively related (Colleran et al. 2015, Skirbekk 2008). New generations of childless women and men can be selected on different factors than earlier ones, and one should account for these factors when assessing the consequence of childbearing on health. E.g., being childless could for recent cohorts be related to a different set of personality traits, mental illnesses or risk behaviours than has been the situation in the recent past. Genetic factors influence personality, mental health and risk behaviour (Linnér et al. 2019, Sanchez‐Roige et al. 2018). Those who are depressed (Doyle and Carballedo 2014) or score high on neuroticism may have fewer children and higher mortality (Deary, Weiss and Batty 2010, Skirbekk and Blekesaune 2013), and these groups may be particularly vulnerable to a lack of social and economic support which a family often provide (Manczak et al. 2018). To understand the causes of childlessness and its implications one needs to account for both social and genetic effects.
Understanding the selection, both for social factors and inherited traits, of those who have fewer or more children is relevant.
Cultural factors matter strongly for fertility choice, and increasingly so in an era with more choice and freedom and less stigma associated to not having children. The decline in fertility and rise in childlessness in many western countries seen in the last few decades relates to changing family norms and value orientations (Dilmaghani 2018, Peri-Rotem 2016). Concepts of family formation can follow social convictions, religious affiliation, views on sexual conduct and family-work life aims. In many societies, having children later in life as well as deciding not to have any children have become more socially permissible (Lesthaeghe 2010, Van de Kaa 2001). A survey of fertility preferences among a relatively homogenous group in terms of income and schooling (women with PhDs from Austria), the share who wanted to be childless was 23% among those without religious beliefs and 7% among those with who were religious (Buber-Ennser and Skirbekk 2016). Cultural differences may therefore help explain why some nations experience much lower fertility levels than others – which in turn may influence health outcomes for these countries.
One topic the centre will focus on is the transition to adulthood in terms of what causes a postponement of fertility -- and health effects thereof. Subjective assessments of age and life stage could be important, including the subjective age when one feels, or is seen as, “mature” and ready for parental responsibilities. For instance, European surveys suggest that what the population see as the average minimum age for having a first child should be around 19 years for women and 21 years for men. Legal age limits affect when one can (and cannot) carry out paid work, marry, have intercourse, or drive a car, factors that may influence lower bounds for partnering and childbearing ages. Socio-economic transitional markers include the age at completing education, the age at leaving parental home, the age of establishing a long term relationship, when one finds a full time job or the age when one buys a house. E.g. the age at first house ownership have increased in many countries in recent decades following higher housing prices. Also, transitions related to type of media and internet use by content or political views may be relevant markers of maturity. Biological phenotype markers include the age at menarche – which has been reduced from 15.5 to below 13 years over the last two centuries in Norway, but also other markers of pubertal transitions, including when one reaches adult height, the age at spermarche or hormone levels. Brain development can affect when cognitive ability reaches its peak, e.g., fluid cognitive abilities (relating to processing novel information) often reach their peak and then decline in early adult years (often in the late 20s), while most personality traits stabilize in early adulthood. Biomarkers may predict fecundity, reproductive behaviour and maturity from DNA and chromosomes (including telomere length, DNA repair, and epigenetic modification), RNA and the transcriptome (transcriptome profiles, circulating microRNAs, and long noncoding RNAs), metabolism (nutrient sensing, protein metabolism, and lipid metabolism), oxidative stress and mitochondria, cell senescence, inflammation, and intercellular communication. The centre should seek to incorporate a broad set of such markers of the transition to adulthood in our studies of fertility and health.
In sum, a lack of interdisciplinary research will often lead to incorrect understanding of the associations between fertility and health. New types of interdisciplinary data, methods – and an open mind-set receptive to alternative explanations and models – can help to significantly improve the quality and novelty of the centre’s research. Social science based research may be less relevant unless integrated with insights from rapidly evolving genomics and neuroscience research. Also, to understand how populations evolve genetically, one needs a thorough analysis of the sociocultural factors influencing fertility.
At the Centre for Fertility and Health we aim to create a research atmosphere that results in fruitful collaborations across disciplines and incentivize research that allows a better understanding of fertility patterns and health outcomes than what each discipline can do alone. The topic of fertility and health is truly interdisciplinary. This necessitates that one goes outside the realm of one’s own area of expertise. Each scientist at the centre should be encouraged to advocate her or his methods, knowledge and hypotheses, but equally important, to acknowledge that other disciplines can be as important – or more important - in answering the relevant research questions.
My hope, when proposing this centre and invited the principal investigators to join, was to create a space that would motivate fertility researchers to go beyond their current lines of research by explicitly integrating relevant insights from both their own discipline, but also routinely incorporating insights from other relevant areas. The proposal was a collaborative effort among leading NIPH scientists with backgrounds in medicine, statistics, economics, demography and genetics, and led to a successful application to the Norwegian Research Council’s Centres of Excellence research scheme. An important factor was the existence in Norway of a multitude of registries and cohorts that include biobanks, giving us realistic chances of resolving the salient research questions we set up. The challenge now is to live up to the expectations we have created.
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