Original ArticlePersonality and long-term reproductive success measured by the number of grandchildren
Introduction
Personality – defined as suites of correlated behavioral tendencies of individuals that are relatively stable across situations and time – has been linked to reproductive outcomes in humans and many non-human animals (Dijkstra and Barelds, 2009, Gurven et al., 2014, Jokela et al., 2009, Jokela et al., 2010, Jokela et al., 2011, Reis et al., 2011, Skirbekk and Blekesaune, 2013, Smith and Blumstein, 2008). In humans, a larger number of children and higher probability of parenthood have been associated with extraversion and related traits, such as sociability and leadership in both sexes (Alvergne et al., 2010, Bailey et al., 2013; Dijkstra and Barelds, 2009, Jokela and Keltikangas-Jarvinen, 2009, Jokela et al., 2009, Jokela et al., 2011), although not all studies have found evidence of these associations (Mealey and Segal, 1993, Nettle, 2005, Perkins et al., 2013). Agreeableness, particularly in women, has also been associated with higher reproductive outcomes (Dijkstra and Barelds, 2009, Jokela et al., 2011). Neuroticism, and its temperamental counterparts high emotionality and harm avoidance, on the other hand, have been associated with lower number of children, delayed parenthood and decreased probability of childbearing (Jokela et al., 2009, Jokela et al., 2010, Jokela et al., 2011, Reis et al., 2011, Gurven et al., 2014) but see (Dijkstra & Barelds, 2009), except in a high-fertility and high-mortality population, where neuroticism appeared to increase number of offspring in women (Alvergne et al., 2010). In addition, one study found that combinations of high neuroticism and low extraversion, or low neuroticism and high extraversion, were associated with higher number of children (Eaves, Martin, Heath, Hewitt, & Neale, 1990).
The two other central personality traits of the widely used Five Factor Model of personality (John, Naumann, & Soto, 2008), conscientiousness and openness to experience, have also been linked to reproductive outcomes, although these findings are fewer in number and less consistent. In different studies, openness to experience has been found to be positively (Gurven et al., 2014, in men), negatively (Jokela et al., 2011), or not at all related to reproductive outcomes (Dijkstra and Barelds, 2009, Alvergne et al., 2010). Conscientiousness and related traits such as persistence have been found both to decrease (Jokela et al., 2010, Jokela et al., 2011) and increase (Dijkstra and Barelds, 2009, Gurven et al., 2014) offspring number. In addition, the negative association between openness to experience and number of children, and the negative association between conscientiousness and number of children in women, appear to have become evident only in cohorts born in wealthy countries in the latter part of the 20th century (Jokela, 2012), suggesting that environmental conditions may moderate the fitness consequences of personality traits. In sum, although several findings on personality and reproductive success have been documented, the specific fitness consequences of different personality traits are yet unclear, and seem to be at least partly associated with differences in the socio-economic development of the studied populations (i.e., high-fertility, high-mortality vs. low-fertility, low-mortality societies).
Most studies on personality traits and reproductive success in humans have focused on offspring quantity. However, life history theory states that individuals may decrease the number of progeny in order to improve their quality (Hill & Kaplan, 1999), that is, increase the survivorship of the offspring or, especially in the case of contemporary Western humans, increase the reproductive success of the offspring. Personality traits have been associated with parental investment styles, e.g. warmth, gentle behavioral control and support for autonomy (Prinzie, Stams, Dekovic, Reijntjes, & Belsky, 2009), and they may influence the reproductive quality of offspring. Thus, the hypothesis is that if personality traits work to steer parental investment toward focusing either on quantity or quality of offspring, the associations between personality and number of children could be nullified or reversed in the generation of the grandchildren. There is some preliminary evidence to support this hypothesis of personality effects on quality–quantity trade-off in non-human animals, although results so far have been mixed. For example, exploratory behavior was not associated with chick provisioning in great tits (Parus major; Patrick & Browning, 2011), whereas more explorative great tits defended their nests with more alarm calls (Hollander, Van Overveld, Tokka, & Matthysen, 2008). In one study with captive minks (Neovison vison), less active females had smaller litters but faster growing kits, suggesting parental investment in the quality of offspring rather than quantity (Meagher, Bechard, & Mason, 2012).
Three studies in humans have tackled the question of personality and quality–quantity trade-offs. In a high-fertility and high-mortality human population, female neuroticism was associated with more offspring but also with lower body condition of the offspring, but this trade-off was only evident in women with low access to resources (Alvergne et al., 2010). Among Tsimane forager-horticulturalists, higher extraversion, conscientiousness and openness to experience, and lower neuroticism in men were associated with higher number of children, but there was no association with personality and children dying before age 15 (Gurven et al., 2014), suggesting no quality–quantity trade-offs. A study of contemporary Americans found interaction effects between offspring number and parental personality on child education: parental neuroticism was detrimental to child education only in large families and parental openness to experience only to later born children (Jokela, Alvergne, Rotkirch, Rickard, & Lummaa, 2014). To date, however, no studies have documented how personality simultaneously predicts offspring quantity and quality in contemporary low-fertility populations when this is assessed in terms of reproductive success in the second generation. Therefore, we do not know how selection acts on personality traits when “fitness” is considered across more than one generation. If personality has antagonistic effects on offspring quality and quantity, the number of children alone may not be enough to assess long-term fitness consequences of different personality traits. Thus one needs to investigate how personality traits are related to number of children and number of grandchildren—which combines both offspring number (quantity) and their survival and reproductive output (quality) into one measure.
We used a large (N = 10,688) representative sample of American women and men in their 60s and 70s to study how personality traits of the Five Factor Model (FFM) were related to number of children and number of grandchildren. The FFM is one of the most widely accepted personality models in humans, and consists of extraversion (sociability, talkativeness, outgoingness), neuroticism (the opposite end of emotional stability), agreeableness (kindness, willingness to cooperate with others), conscientiousness, and openness to experience (intellect, creativeness) (John et al., 2008). These data allowed us to investigate (i) how personality is related to number of children, (ii) whether and how personality is related to number of grandchildren, and (iii) whether there are differences in the relations between number of children and number of grandchildren that could be indicative of the existence of quality–quantity trade-offs. In addition, we tested whether sex affected the above mentioned associations in order to determine whether personality is under similar selection in both sexes.
Section snippets
Participants
The data were derived from the Health and Retirement Study (HRS), which is a nationally representative longitudinal study of more than 30,000 individuals representing the U.S. population older than 50 years (Juster & Suzman, 1995). Telephone or in-person interviews are conducted every second year, administered under the National Institute of Aging (NIA) and the University of Michigan's Institute for Social Research. As of 1992, the HRS consists of many different sources of data collection, and
Number of children
Higher extraversion, lower conscientiousness, and lower openness to experience were associated with higher numbers of children in both sexes (Supplementary Table S1), with no sex differences in any of the associations (all interactions between sex and personality traits were non-significant, p-values > .05). Compared to people with low extraversion (− 1 standard deviation, SD, below the mean), people with high extraversion (+ 1 SD above the mean) had 0.15 (5.6%) more children; compared to people
Discussion
We found that personality traits of the Five Factor Model were similarly associated with both number of children and number of grandchildren in a large sample of contemporary U.S. citizens. Larger number of children and grandchildren were associated with higher extraversion, lower conscientiousness and lower openness to experience. In addition, higher agreeableness was associated with larger number of grandchildren, but not with number of children, but the difference between these associations
Supplementary Materials
The following are the supplementary data to this article.
Acknowledgments
This research was supported by the Kone Foundation, Research Funds of the University of Helsinki, and Academy of Finland (266898).
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