Compared to vegans, meat consumers experienced both lower depression & anxiety; the more rigorous the study, the more positive and consistent the relation between meat consumption and better mental health

Meat and mental health: A meta-analysis of meat consumption, depression, and anxiety. Urska Dobersek et al. Critical Reviews in Food Science and Nutrition, Oct 6 2021. https://www.tandfonline.com/doi/full/10.1080/10408398.2021.1974336

Abstract: In this meta-analysis, we examined the quantitative relation between meat consumption or avoidance, depression, and anxiety. In June 2020, we searched five online databases for primary studies examining differences in depression and anxiety between meat abstainers and meat consumers that offered a clear (dichotomous) distinction between these groups. Twenty studies met the selection criteria representing 171,802 participants with 157,778 meat consumers and 13,259 meat abstainers. We calculated the magnitude of the effect between meat consumers and meat abstainers with bias correction (Hedges’s g effect size) where higher and positive scores reflect better outcomes for meat consumers. Meat consumption was associated with lower depression (Hedges’s g = 0.216, 95% CI [0.14 to 0.30], p < .001) and lower anxiety (g = 0.17, 95% CI [0.03 to 0.31], p = .02) compared to meat abstention. Compared to vegans, meat consumers experienced both lower depression (g = 0.26, 95% CI [0.01 to 0.51], p = .041) and anxiety (g = 0.15, 95% CI [-0.40 to 0.69], p = .598). Sex did not modify these relations. Study quality explained 58% and 76% of between-studies heterogeneity in depression and anxiety, respectively. The analysis also showed that the more rigorous the study, the more positive and consistent the relation between meat consumption and better mental health. The current body of evidence precludes causal and temporal inferences.

Keywords: anxietydepressionmeatmental healthveganvegetarianismsex

Discussion

This meta-analysis extends the findings of our prior systematic review (Dobersek et al. 2020) by presenting a quantitative evaluation of the relation between meat consumption/abstention and mental health. It included 171,802 participants aged 11 to 105 years, from varied geographic regions, including Europe, Asia, North America, and Oceania. The findings show a significant association between meat consumption/abstention and depression and anxiety. Specifically, individuals who consumed meat had lower average depression and anxiety levels than meat abstainers. We also showed that vegans experienced greater levels of depression than meat consumers. Sex did not modify these relations. Study quality explained a significant proportion of between-studies heterogeneity and a cumulative meta-analysis confirmed these findings. Specifically, the higher the study quality, the more positive the benefit of meat consumption.

Our results may explain the equivocal nature of prior research. In contrast to our clear findings (both past (Dobersek et al. 2020 and present), other systematic reviews and meta-analytic results were inconsistent or contradictory. These equivocal results suggested that vegetarians, and in some cases vegans had lower levels of depression or anxiety (Askari et al. 2020; Iguacel et al. 2020; Lai et al. 2014; Li et al. 2017; Liu et al. 2016; Nucci et al. 2020; Zhang et al. 2017). As detailed in our systematic review (Dobersek et al. 2020), numerous factors explain these inconsistent conclusions. Briefly, most prior studies employed invalid or unreliable assessment protocols to measure exposures and outcomes (i.e., diet and mental health, respectively). For example, it is well established that dietary recalls and FFQs produce physiologically implausible and non-falsifiable (pseudo-scientific) data (Archer, Pavela, and Lavie 2015; Archer, Hand, and Blair 2013; Archer, Lavie, and Hill 2018a; Archer, Marlow, and Lavie 2018b, 2018c). Thus, the disparity between self-reported and actual dietary intake may render definitive conclusions impossible when analyzing meat consumption as a continuous rather than dichotomous variable (Archer, Pavela, and Lavie 2015; Archer, Hand, and Blair 2013; Archer, Lavie, and Hill 2018a; Archer, Marlow, and Lavie 2018b, 2018c).

With respect to mental health, the most rigorous research relied on physician-diagnosed disorders using the Diagnostic and Statistical Manual of Mental Disorders (DSM) (Michalak, Zhang, and Jacobi 2012) (APA 2013) rather than self-reported (subjective) assessments with untested validity. The use of tools with questionable validity can lead to ambiguous findings and limited cross-study analyses.

Another major design error was the use of biased and selective sampling strategies. Several of the included studies recruited samples from vegan and vegetarian websites, social-networking groups, communities, and restaurants. We surmise this may have substantially biased data collection and may skew self-reported variables and findings if participants with a high degree of emotional or ideological commitment to their dietary behaviors intentionally or unconsciously misreport. An antecedent of this error may be a form of confirmation bias in which the flawed sampling confirms the investigators’ ideology or expectations rather than providing dispassionate data and results.

Finally, statistical and communication errors were ubiquitous. These included the failure to correct for multiple comparisons and the inappropriate use of causal language which can lead to invalid results, interpretations, and conclusions. In summary, given that these errors are widespread in the literature, valid conclusions from previous reviews that failed to examine study quality are not possible.

In the present meta-analysis, these errors taken together are related to significant between-studies variation in effect sizes. Study quality explained 58% and 76% of between-studies heterogeneity in the differences in depression and anxiety, respectively. Furthermore, our analyses (see Figures 2, 4, 6, 7, 10, and 11) demonstrated that higher quality studies showed a more positive and consistent relation between meat consumption and mental health. Higher quality studies had much larger sample sizes.

Finally, limited reporting of participant characteristics prevented an examination of several covariates (e.g., BMI, age of diet adoption/length of diet, clinical history, socioeconomic status, culture) that could potentially contribute to between-studies heterogeneity.

Strengths and limitations

Strengths

This meta-analysis had several strengths. First, our a priori decision to select only studies that provided a clear dichotomy between meat consumers and meat abstainers allowed for a clear and rigorous assessment. While myriad studies have examined vegetarianism along a continuum, these were excluded because the lack of a clear distinction between groups rendered inferences equivocal. This distinction is necessary because self-reported (memory-based) dietary assessments (FFQ) should not be used for quantitative analyses because of their invalidity. Any study that attempts to use FFQs as continuous variables are invalid due to nonquantifiable measurement error (Archer, Lavie, and Hill 2018a; Archer, Marlow, and Lavie 2018b; Archer, Pavela, and Lavie 2015).

Second, we limited our psychological outcomes to the most prevalent and debilitative disorders: depression and anxiety. This allowed a focused yet rigorous analysis and ameliorated the effects of poorly operationalized psychological phenomena such as disordered eating, dietary restraint, orthorexia, and neuroticism. This exclusion helps to avoid potential misclassification and concomitant pathologizing of those who simply wish to avoid specific foods or food groups (e.g., vegans). Finally, with over 170,000 participants from several geographic regions, our meta-analysis allowed for more generalizable and definitive conclusions.

Limitations

Our meta-analysis also had limitations. First, we excluded non-English-language studies. This potentially biased our results in favor of ‘Western’ norms which include meat consumption. For example, we excluded papers published in languages other than English (e.g., Japanese, Hindi). Thus, we may have omitted studies from geographic regions that follow predominantly vegetarian or plant-based dietary patterns.

Second, while our search was clearly defined and comprehensive, our inclusion criteria excluded many publications that provided data on this topic (e.g., see (Anderson et al. 2019; Barthels, Meyer, and Pietrowsky 2018; Burkert et al. 2014; Cooper, Wise, and Mann 1985; Jacka et al. 2012; Larsson et al. 2002; Li et al. 2019; Northstone, Joinson, and Emmett 2018)). Specifically, these papers were excluded because they examined constructs other than depression or anxiety (e.g., orthorexia, restrained eating behavior) or assessed meat consumption as a continuous rather than dichotomous variable. As previously stated, self-reported dietary status and FFQs lead to nonquantifiable measurement error. Nevertheless, we think that our rigorous and highly focused meta-analysis has the potential to provide stronger evidence for the medical, research, and lay communities.

Third, despite the high confidence we place in our finding that meat abstention is linked to a greater prevalence of psychological disorders, study designs precluded inferences of temporality and causality. Specifically, only two of the included studies (Lavallee et al. 2019; Velten et al. 2018) provided information on temporality. Therefore, we were unable to conclusively examine this effect. Given that there are many reasons why people abstain from meat (e.g., ethical, environmental, animal rights-related reasons), this empirical question has not been adequately addressed. However, our previous systematic review (Dobersek et al. 2020) showed conflicting evidence on the temporal relations between meat abstention and depression and anxiety. Also, conclusions on causality require evidence from rigorous RCTs. Since only one low-quality RCT met our inclusion criteria (Beezhold and Johnston 2012), no conclusions regarding causality are supported.

Finally, the results of our meta-analysis are only as valid as the data collected in the included primary studies. Given that most studies used FFQs and self-reported questionnaires, participants may have been misclassified. Merely reporting that one avoids meat is not the equivalent of actual meat abstention (Archer, Pavela, and Lavie 2015; Archer, Hand, and Blair 2013; Archer, Lavie, and Hill 2018a; Archer, Marlow, and Lavie 2018b, 2018c). In fact, self-defined vegetarians and meat abstainers may consume meat (Haddad and Tanzman 2003).

Recommendations for future directions

Future investigators should avoid the most common flaws exhibited in the included studies. First, investigators must acknowledge and address the effects of both researcher and participant biases (e.g., confirmation bias, cognitive dissonance, observer-expectancy effects/reactivity) when employing highly selective or biased samples. Individuals who are highly invested in their dietary behaviors may be predisposed to intentional and non-intentional misreporting.

Second, the use of physician-diagnosed disorders based on criteria from the DSM-V (APA 2013; Michalak, Zhang, and Jacobi 2012) is preferable to self-reported symptoms, and assists in producing more definitive results. Additionally, the severe limitations and pseudo-scientific nature of self-reported dietary data and FFQs (Archer, Pavela, and Lavie 2015; Archer, Hand, and Blair 2013; Archer, Lavie, and Hill 2018a; Archer, Marlow, and Lavie 2018b, 2018c) could be overcome in part with point-of-purchase (barcode) data (Ng and Popkin 2012). However, while these data may be less biased, they are not necessarily an accurate proxy for actual dietary consumption.

Third, the use of more rigorous study designs (e.g., RCTs) is desirable over mere observational investigations. However, it would be extremely difficult to conduct a randomized study of diets with a long enough duration to impact fundamental affective outcomes such as anxiety and depression. Furthermore, detailed participant information regarding behavioral and health-related histories and current lifestyles is essential to valid interpretation and conclusions. Finally, studies should provide complete statistical information that allow for the calculations of effect sizes. More complete reporting would enable meta-analysts to extract both effect measures and study characteristics thus allowing for exploration of potentially important but unanswered questions (e.g., how is time of diet adoption related to mental health?).

Heterozygosity of the major histocompatibility complex predicts later self-reported pubertal maturation in men, suggesting a genetic trade-off between immunocompetence and sexual maturation in human males

Heterozygosity of the major histocompatibility complex predicts later self-reported pubertal maturation in men. Steven Arnocky, Carolyn Hodges-Simeon, Adam C. Davis, Riley Desmarais, Anna Greenshields, Robert Liwski, Ellen E. Quillen, Rodrigo Cardenas, S. Marc Breedlove & David Puts. Scientific Reports volume 11, Article number: 19862. Oct 6 2021. https://www.nature.com/articles/s41598-021-99334-5

Abstract: Individual variation in the age of pubertal onset is linked to physical and mental health, yet the factors underlying this variation are poorly understood. Life history theory predicts that individuals at higher risk of mortality due to extrinsic causes such as infectious disease should sexually mature and reproduce earlier, whereas those at lower risk can delay puberty and continue to invest resources in somatic growth. We examined relationships between a genetic predictor of infectious disease resistance, heterozygosity of the major histocompatibility complex (MHC), referred to as the human leukocyte antigen (HLA) gene in humans, and self-reported pubertal timing. In a combined sample of men from Canada (n = 137) and the United States (n = 43), MHC heterozygosity predicted later self-reported pubertal development. These findings suggest a genetic trade-off between immunocompetence and sexual maturation in human males.

Discussion

Our results support the prediction that greater MHC heterozygosity, a genetic contributor to pathogen resistance33,34, predicts later pubertal timing. In a combined data set derived from two independent samples, MHC heterozygosity predicted relative, but not absolute, recalled puberty. Because males lack a salient, singular pubertal event like menarche, when considering retrospective reports relative pubertal timing may be more accurate because men may be better able to recall whether they matured earlier or later than their peers rather than the precise ages of pubertal events48,49.

Correlations between immunocompetence and pubertal timing could reflect the linked heritability of both traits, common developmental underpinnings50, or pleiotropic effects of MHC genes, which could influence both immunocompetence and sexual maturation. Indeed, some research has shown that MHC class II expression occurs alongside maturation of the adrenal cortex51. Interestingly, dehydroepiandrosterone (DHEA), which is produced by the adrenal cortex and affects aspects of reproductive development, has been implicated in immune function in humans and other species52,53.

LHT offers a framework to explain why immunocompetence and pubertal timing may be related at a functional level: Individuals with reduced extrinsic mortality risk due to lower vulnerability to pathogens may be able to continue growth and delay sexual maturation and reproduction. If so, then selection should favor mechanisms, potentially including pleiotropy and genetic linkage, that couple immunocompetence and the timing of sexual maturation. This possibility aligns with some research on intra-species differences in LH. For example, Tasmanian devil populations affected by an infectious facial tumor disease had a 16-fold higher chance of reaching sexual maturity at an earlier age than usual26. In a study of 22 small-scale human societies, populations with higher extrinsic mortality risk displayed earlier puberty and reproduction—as well as shorter adult height and life expectancy5.

Future work must reconcile research showing opposing patterns, such as among perinatal HIV infection and slower pubertal maturation54. Perhaps the distinction lies in genetic versus acquired factors affecting immunocompetence, or environmental factors (e.g., food energy availability of safety/survival rates), which might also influence luteinizing hormone (LH) release in diverse human populations55. For example, malnutrition has been linked to delayed pubertal maturation in humans44. Accordingly, future research should consider the role of energy availability in the environment as a potentially important moderator of the potential link between MHC and pubertal development. For instance, perhaps the influence of infectious burden on energy availability may be lower in populations with energy abundance and substantial health infrastructure, such as in Western industrialized nations.

Our findings also help explain why MHC heterozygous men have been found to be taller in adulthood56. Height is driven by long bone growth via chondrogenesis at the growth plate57, and epiphyseal fusion at puberty terminates growth. Later pubertal maturation allows more long bone growth before epiphyseal fusion, resulting in taller adult height18; therefore, heterozygous individuals may be taller because they begin puberty later. Future research could test whether pubertal timing mediates the relationship between MHC heterozygosity and adult height. It may be useful to examine the potential moderating role of early stressors in the environment to the MHC-pubertal timing link. From this perspective, developmental plasticity gives rise to an array of phenotypes that emerge in response to specific local social and ecological conditions58. These genetic variants are putatively adaptive insofar as they contribute to greater fitness in the environments in which they manifest. Accordingly, an interaction between HLA homozygosity and early life stressors may be a stronger predictor of pubertal timing than either variable alone.

Within the context of LHT, some researchers have predicted that greater investment in immunocompetence should correspond with later sexual maturation. Although previous research linking early pubertal maturation to a diverse range of health problems supports this notion, this is the first research to demonstrate a correlation between MHC heterozygosity and later recalled pubertal development. Such a link has important implications for understanding the development of puberty-linked physical and mental health outcomes. These results suggest that variation in genetic influences on pubertal timing may reflect a trade-off between somatic growth and maintenance and reproduction, at least in energy-rich environments. However, within the broader context of well-established positive links between environmental condition and earlier (rather than later) pubertal timing, these findings imply that understanding variability in reproductive effort will likely rely upon examining more complex interactions between genetics and local ecological condition.

The dominant theory of facial attractiveness judgments is that they evolved to identify healthy individuals with strong immune systems; this paper finds little compelling empirical support

Does facial attractiveness really signal immunocompetence? Benedict C. Jones, Iris J. Holzleitner, Victor Shiramizu. Trends in Cognitive Sciences, October 5 2021. https://doi.org/10.1016/j.tics.2021.09.003

Abstract: The dominant theory of facial attractiveness judgments is that they evolved to identify healthy individuals with strong immune systems. Here, we summarize results of recent tests of this hypothesis, concluding that it has little compelling empirical support. We then propose an alternative perspective that emphasizes the effects of lifestyle health.

Human male reproduction exhibits distinct and derived features: Long-term reproductive partnerships, intensive paternal care including provisioning, shortened interbirth intervals, and later age-specific fertility than great ape comparisons

Men and reproduction: Perspectives from biological anthropology. Peter B. Gray, Alex Straftis, Kermyt G. Anderson. Chp in The Routledge Handbook of Anthropology and Reproduction, Routledge. ISBN 9781003216452. https://www.taylorfrancis.com/chapters/edit/10.4324/9781003216452-5/men-reproduction-peter-gray-alex-straftis-kermyt-anderson

Abstract: Charles Darwin left an intellectual legacy to human evolution and sexual selection that still frames our understanding of men and reproduction. Here, we take an integrative approach to men and reproduction and, like Darwin and his intellectual descendants, seek to incorporate evolutionary theory, comparative evidence, a life course approach, physiology, and human diversity. We discuss theoretical foundations, such as life history tradeoffs and proximate vs. ultimate causation. We turn to a model of male reproduction that highlights age-specific fertility and multiple modes of paternal contributions. Human male reproduction exhibits distinct and derived features: Long-term reproductive partnerships, intensive paternal care including provisioning, shortened interbirth intervals, and later age-specific fertility than great ape comparisons. Male contributions can include genetics, epigenetics, protection, provisioning, direct child care, and social transmission. While male investment is often sensitive to paternity and paternity certainty, stepfathering indicates other motivations beyond genetic paternity can shape investment. We discuss developmental, male–male competition, and female choice contributions to male reproduction. We then turn to consequences of reproduction for men, highlighting time allocation, relationship dynamics, physiological, health, and other primary impacts. Last, we touch on the consequences of men’s reproduction for other adults such as mates and for men’s children.

Female bonobos engaged more in genito-genital rubbing when their sexual swelling was in the maximum phase, exchanging grooming & sex according to the daily “market fluctuations” associated with swelling status

Sex and grooming as exchange commodities in female bonobos’ daily biological market. Simone Anzà, Elisa Demuru & Elisabetta Palagi. Scientific Reports volume 11, Article number: 19344. Sep 29 2021. https://www.nature.com/articles/s41598-021-98894-w

Abstract: The Biological Market Theory (BMT) posits that cooperation between non-human animals can be seen as a mutually beneficial exchange of commodities similarly to what observed in human economic markets. Positive social interactions are commodities in non-human animals, and mutual exchanges fulfilling the criteria of the BMT have been shown in several species. However, the study of biological markets suffers from methodological limitations that are mainly linked to the difficulty of clearly identifying the currencies and their exchanges in the short-term. Here, we test whether bonobo females are more attractive during their maximum swelling phase, whether they exchange grooming and Genito-Genital Rubbing (GGR) on a daily level of analysis, and whether these daily exchanges fulfil the BMT criteria. Females engaged more in GGR when their sexual swelling was in the maximum phase. Moreover, they exchanged grooming and sex according to the daily “market fluctuations” associated with swelling status. Females in the minimum phase (low-value) increased their probability to engage in GGR with females in the maximum phase (high-value) by grooming them preferentially. In line with the supply/demand law, the female grooming strategy varied depending on the daily number of swollen females present: the higher the number of swollen females, the lower the individual grooming preference. As a whole, our study confirms BMT as a valid model to explain daily commodity exchanges as a function of the temporary value of traders, and underlines the importance of a day-by-day approach to unveil the presence of a biological market when the value of traders frequently changes.

Discussion

Taken together, our results confirm the key role of sexual swelling in bonobos and provide new insights on the exchange of grooming with sex on a daily basis. More particularly, our study shows that grooming and sex represent valuable exchange commodities for bonobo females and that their exchange fluctuates depending on females’ sexual swelling status. These daily fluctuations can be explained by the supply/demand law1. In line with other studies on sexual swelling and socio-sexual behaviour in bonobos35,36, our results confirmed that females engaged significantly more in Genito-Genital Rubbing (GGR) when at least one female of the dyad was in the maximum swelling phase, compared to when both of them were in the minimum swelling phase (Prediction 1). When in the maximum swelling phase, females are more attractive for both males and females and tend to be invited more frequently to engage in sexual interactions than females in the minimum swelling phase26. This is in line with the hypothesis proposing that the peculiar features of bonobo females’ sexual swelling, and in particular its extremely long duration and presence during anovulatory periods, might have been selected to increase female-female sexual interactions35. Socio-sexuality in bonobo females help establishing and maintaining strong social bonds which, in turn, allow them to form coalitions that increase their social status and centrality30.

According to the Biological Market Theory derived from the Market Theory applied in economics, the exchange rate of commodities depends on the supply/demand law, and when a certain commodity becomes common, its perceived value decreases. Consistent with the supply/demand law, the daily grooming preference was significantly affected by the number of females showing the sexual swelling at the maximum phase during the same given day: the higher the number of swollen females, the lower the daily grooming preference (Prediction 2a, Fig. 1). Interestingly, when both the actor and the receiver were in the minimum swelling phase, and there were no females available in the maximum swelling phase, the grooming preference was very high. Via Model 1 we considered the directional grooming preference and we included the preference of both dyads AB and BA. Therefore, our results suggest that females exchanged grooming for grooming when both in the minimum swelling phase. Moreover, the significant effect of the interaction swellingact*swellingrec showed higher grooming preference when both the actor and the receiver were in the maximum swelling phase, compared to when the actor was in the minimum and the receiver was in the maximum swelling phase. These results confirm the higher attractiveness of females showing the maximum swelling phase (Prediction 1, 2b), suggest that females tend to exchange grooming for grooming when their value is “low” (MIN) and that high-value females (MAX) prefer “high-value” grooming partners as well.

Our results on the effect of the number of swollen females on grooming preference are in line with a study on male–female grooming in captive chimpanzees11 showing that females in the maximum swelling phase receive more grooming. Although our investigation focussed on female-female sexual interactions, our results are in line with those of chimpanzees given that the highest daily preference was achieved when both actor and receiver were in the maximum swelling phase. The results coming from the two Pan species are comparable considering the high attractiveness of the maximum sexual swelling in both chimpanzees and bonobos although in chimpanzees the sexual swelling is attractive for males only, while in bonobos this signal is attractive for both sexes.

In many primate groups, higher-ranking subjects typically receive more grooming than lower-ranking ones51,52,53 and most of the grooming occurs between subjects with similar rank54. Among bonobos, the distribution of grooming is related to several variables (e.g., age, rank, and sex) and there are some indications that high-ranking and older females are preferred grooming partners55. Subjects within the same group may therefore compete for grooming access to high-ranking groupmates, since those can be the best coalition partners52,54. Alternatively, a grooming distribution related to rank can be explained by females’ preference in grooming subjects of similar social class (e.g., rank and age) as an adaptive strategy derived from greater compatibility between subjects56. These mechanisms are beyond the aim of our investigation but can still drive the variation of grooming preferences and mask the effect of the daily market. For this reason, we included in our models rank and age of both actor and receiver as control predictors. However, our results on the daily grooming preference showed no effect of actor and receiver’s rank and age, suggesting that the factors shaping grooming preference among bonobo females on a daily basis may not coincide with those affecting such preference on the long term. Moreover, it has been showed that the steepness of the hierarchy varies between different bonobo groups in captivity20,25. Particularly, these studies demonstrated that grooming was preferentially directed towards dominant individuals in those groups where the hierarchy was very steep, whereas grooming was more reciprocal in those groups showing a mild hierarchy. It is worth noting that the steepness of our study groups was extremely low (Apenheul: 0.346; La Vallée: 0.305 unpublished data).

The daily occurrence of GGR was significantly affected by the number of females in the maximum swelling phase: the higher the number of swollen females, the lower the daily occurrence of GGR (Prediction 3a). Post-hoc analyses revealed no significant effect of grooming preference on the daily occurrence of GGR when the actor was in the maximum swelling condition and the receiver was in the maximum or minimum swelling condition (Fig. 3b). On the other hand, when an actor in the minimum swelling phase preferentially groomed a receiver in the maximum swelling phase, it significantly increased the probability of engaging in GGR compared to when the receiver was in the minimum swelling phase (Prediction 3b, Fig. 3a). This might indicate that: (1) when both partners are in the minimum swelling phase, none of them has the “economic power” to influence the exchange of goods and grooming is therefore not exchanged with sex (Fig. 3a; MIN-MIN), and that (2) lower-value females (MIN) need to provide more grooming to get the access to higher-value partners (Fig. 3a; MIN–MAX). However, this strategy seems to work only when is the receiver (of grooming/sexual invitations) who benefits from the mismatch, especially if we consider that an increase of grooming performed by a high-value actor does not increase the chances of engaging in sexual interactions: a high-value actor has a constant chance of engaging in sexual interactions despite receiver’s value (Fig. 3b; MAX–MIN, MAX-MAX). The results from Model 1 and Model 2 are consistently stressing that grooming can be performed to balance the mismatch in females’ value, and when the partners have equal value, grooming can be exchanged for grooming as well.

Females in the minimum swelling phase exhibit a short-term strategy of exchanging commodities shaped by the supply/demand law and receiver’s value. Given that females in the maximum swelling phase are more sexually (and therefore socially) attractive, females in the minimum swelling phase must augment the commodity they provide to increase their possibility of engaging in sexual interactions with higher-value females. Furthermore, by strategically offering more grooming, lower-value females can even outcompete higher-value ones to gather “sexual favours” from the most attractive partners.

In conclusion, our study confirms the BMT in explaining the daily exchange of commodities and its fluctuations as a function of the temporary value of traders, and underlines the importance of a day-by-day approach to unveil the presence of a biological market when the value of traders rapidly changes.

Americans perceived political ingroups as more prototypical of Americans; perceiving the outgroup as poorly fitting the prototype of the superordinate group predicted opposition to bipartisan cooperation and a lower likelihood of bipartisan behavior

Ingroup Projection in American Politics: An Obstacle to Bipartisanship. Angela C. Bell et al. Social Psychological and Personality Science, October 5, 2021. https://doi.org/10.1177/19485506211046788

Abstract: One potential obstacle to cooperation between political parties is ingroup projection, the tendency for members of subgroups to define superordinate groups based on characteristics of their own ingroups. In five studies spanning 11 years and three presidential administrations, we demonstrated that ingroup projection can be an obstacle that prevents bipartisanship between Republicans and Democrats. Study 1 showed that Americans perceived political ingroups as more prototypical of Americans than outgroups and that the degree of mismatch between the outgroup and the superordinate group was associated with ingroup bias. Studies 2–5 demonstrated that perceiving the outgroup as poorly fitting the prototype of the superordinate group predicted opposition to bipartisan cooperation and a lower likelihood of having engaged in bipartisan behavior (Studies 4 and 5). These studies provide evidence for ingroup projection among American political parties and suggest that it contributes to political polarization.

Keywords: bipartisanship, ingroup projection, polarization, common ingroup identity, social identity theory

Stability and change in major life goals during the transition to parenthood: Little evidence that parenthood leads to change in major life goals

Stability and change in major life goals during the transition to parenthood. Caroline Wehner, Manon A. van Scheppingen, Wiebke Bleidorn. European Journal of Personality, March 1, 2021. https://doi.org/10.1177/0890207021996894

Abstract: The transition to parenthood introduces changes in various life domains. In this paper, we examined whether and to what degree the transition to parenthood is related to changes in the importance of major life goals. To do this, we examined the rank-order stability, ipsative stability, and mean-level change in six life goal domains (achievement, power, variation, affiliation, altruism, and intimacy) in a sample of 248 parents and 294 individuals in a romantic relationship without children across two time points. Overall, we found high rank-order (variable-oriented) and ipsative (person-oriented) stability, and little evidence for mean-level changes in the importance of life goals across the transition to parenthood. However, we found several selection effects suggesting that women without children tended to endorse agentic life goals (variation and achievement) more than mothers did. Generally, our findings underline the overall stability of life goals and their role as guiding principles in life.

Keywords: life goals, development, parenthood, period of transition, life event

Discussion

In the present study, we observed stability and change of life goals in first-time parents and couples without children. We examined stability using a variable-oriented (rank-order stability) and a person-oriented approach (ipsative stability). Both indices indicated high stability in parents’ and nonparents’ life goals over the course of one year. Expecting mothers but not fathers differed from nonparents in several goal domains prior childbirth (selection effects). However, we found little evidence for mean-level change in general, or for differences in the mean-level change trajectories of parents and nonparents (socialization effects). Overall, these results suggest that the importance of major life goals is quite stable, even amid a major life event such as the transition to parenthood. As such, selection rather than socialization effects appear to explain the differences in life goals between parents and nonparents.

How stable are life goals?

The rank-order stability of all goal dimensions was high and in the range of previous findings (e.g., r = .65 – .75 in Lüdtke et al., 2009). There were only few differences between parents and nonparents. Mothers appeared to be less rank-order stable than nonmothers in their endorsement of variation, and fathers appeared less stable than nonfathers in achievement. These results are consistent with our hypothesis of lower stability in parents. Notably, both variation and achievement are agentic goals which leads to the question why stability in parents tends to be lower in agentic but not in communal goals. One explanation may be that communal goals are generally more stable, which may express itself in transitional periods such as parenthood or in the long-term. Atherton et al. (2020) found some evidence that goals’ stability tends to be higher in communal goals (family/relationship r = .58, social r = .44) than in agentic goals (economic r = .36; but political r = .46) across 24 years. Overall, however, parenthood had a smaller effect on the rank-order stability of life goals than we expected given the profound impact parenthood entails on people’s resources, life style, and routines. Notably, the limited sample size in each group resulted in relatively large confidence intervals which made their comparison a rather conservative test.

Not only the rank-ordering of individuals along the goal dimensions but also the rank-ordering of goal dimensions within individuals – the ipsative stability of life goals – was high in both parents and nonparents. That means, independent of the parenthood status for most individuals the order in the importance of their life goals did not change across the study period. However, we explored and found significant differences between women and men indicating that even though both genders were highly stable, men were slightly less consistent in the ranking of their life goals.

In summary, results for both indicators of stability, variable- and person-oriented, emphasized that life goals are rather stable constructs, even in the face of a major life event such as the transition to parenthood. This finding is consistent with the life goals literature that has highlighted their function as guiding principles that provide direction and consistency across situations and roles (Hennecke & Freund, 2017; Roberts & Robins, 2000).

However, high levels of rank-order and ipsative stability do not exclude the possibility of mean-level change in major life goals. To test whether life goals change during the transition to parenthood, we compared parents’ and nonparents’ life goals before and after the transition to parenthood.

Do parents and nonparents differ in their life goals?

As hypothesized, we found evidence for selection effects indicating that mothers and nonmothers differ in their life goals. Specifically, compared to nonmothers, mothers scored generally lower in the agency-related goals achievement and variation. This finding is consistent with our hypothesis that expecting mothers prioritize specific life goals. Indeed, the transition to parenthood might reflect certain goals, such as the idea to settle down, i.e. allows for less variation. The robust selection effect of low openness to experience (e.g., van Scheppingen et al., 2016), which is among others defined by a preference for variety and curiosity, is also in line with our finding of less variation in mothers. In qualitative interviews, Bass (2015) found that women tended to downshift their career goals, i.e. achievement, in anticipation of parenthood. Men, in comparison, usually did not do so, which is in line with our finding of no differences between fathers and nonfathers. This might be particularly pronounced in our Dutch sample, as the Netherlands have by far the highest rate of part-time employment in women compared to their neighboring countries (OECD, 2019) and a relatively short leave for fathers. We further hypothesized but found no selection effects for communal goals. As pointed out above, communal goals might be more stable than agentic goals (Atherton et al., 2020) and thus are less prone to change in general.

Is parenthood associated with change in life goals?

Previous research established age-graded mean-level decreases in life goal importance, particularly during young adulthood (Atherton et al., 2020; Roberts et al., 2004). This pattern might be a result of age-graded decreases in resources (Lüdtke et al., 2009; Roberts et al., 2004) that tend to occur as a consequence of life events such as the transition to parenthood. Goal selection, i.e. decrease in most life goals’ importance, which is assumed to ensure functioning and effective goal pursuit (Baltes et al., 2006), may be accelerated by major life events. Consistent with these assumptions, we expected mean-level decreases in most life goals, particularly in parents, as they should experience an even greater decrease in resources than nonparents (see for example Nelson et al., 2014). Different to our expectations, we found very little evidence for mean-level change in life goals and only few differences between parents and nonparents.

In summary, we found little evidence for life goal change in response to the transition to parenthood. The lack of socialization effects associated with parenthood is consistent with research on other individual difference variables, such as the Big Five (Galdiolo & Roskam, 2014; Jokela et al., 2011; van Scheppingen et al., 2016; but see also Asselmann & Specht, 2020). Overall, life goals and other psychological variables appear to predict who will select into parenthood but do not seem to change in response to the experience of this life transition.

An important question is when the observed differences between parents and nonparents emerge. For example, parents-to-be may undergo changes in life goals long before their decision of having a child (see Bass, 2015) or during pregnancy. Salmela-Aro et al. (2000) found some evidence to support the hypothesis that parents’ experience changes in life goals immediately before the birth of their child, at least with regard to mothers’ achievement strivings. While the amount of achievement goals declined from the 12th to the 36th week of pregnancy in this study, it was stable from the 36th week to 3 months after birth. In the present study, life goals were only assessed once before child birth, thus we could not quantify the timing of change.

Gender differences

We hypothesized that parenthood would have a more profound influence on mothers’ than fathers’ goal structures, as mothers experience generally more physical, emotional, and psychological changes and often assume the role of the primary caregiver (Poudevigne & O’Connor, 2006; Ruppanner et al., 2019; Salmela-Aro et al., 2000). Even though we found only few effects, mothers tended to differ more in life goals from nonmothers, than fathers did from nonfathers. In particular, we found several selection effects in women’s agentic life goals but none in men’s. This might be explained by anticipation effects. That is, Bass (2015) found that women but not men tend to constrain their career aspirations in anticipation of potential future parenthood. She suggested that socially constructed gender expectations may be a driving force leading to more mental examination of possible role conflicts, time and financial constraints occurring with parenthood, which may ultimately lead to lower scores in agency related goals in becoming mothers. Gender differences may also be explained by selection, based on more stable individual differences in goals, instead of anticipation. For men who are high on agentic goals (in the Netherlands), it might not make a big difference for their career if they have children or not, as they just keep working. For women it does make a difference, so women high on agentic goals may be more likely to postpone childbirth or not have children at all.

Limitations and future directions

The strengths of this pre-registered study include the use of a prospective longitudinal design including parents and nonparents which allowed us to examine stability and change in life goals during the transition to parenthood. Nevertheless, there are several limitations that should be considered. First, even though the total sample consisted of more than 500 participants, the sample size within the groups of mothers, fathers, nonmothers, and nonfathers was only modest by contemporary standards resulting in limited statistical power. Second, studies with more and earlier assessments leading up to the decision of having a baby could provide important insights into potential anticipation effects prior and during pregnancy. With our data, we were not able to detect such potential early changes. Third, we found attrition effects for power indicating that participants who dropped out scored higher in this domain compared to participants who remained in the study. Moreover, the latent change score models for power did not fit the data well suggesting that the results for this goal domain should be interpreted with caution. Fourth, parents were on average about four years older than nonparents. These limitations emphasize the need for replication of our results in future research. Beyond that it would be interesting to not only look at goal importance but also into the effort people tend to put into reaching their goals. The interplay of goal importance and goal effort could help to further understand the development of life goals (see Atherton et al., 2020 for a similar reasoning).