This study explores directional selection on physical and psychosocial phenotypes in Eastern Eurasian populations, utilizing a dataset of 1245 ancient genomes. By analyzing polygenic scores (PGS) for traits including height, educational attainment (EA), IQ, autism, schizophrenia, and others, we observed significant temporal trends spanning the Holocene era. The results suggest positive selection for cognitive-related traits such as IQ, EA and autism spectrum disorder (ASD), alongside negative selection for anxiety and depression. The results for height were mixed and showed nonlinear relationships with Years Before Present (BP). These trends were partially mediated by genetic components linked to distinct ancestral populations. Regression models incorporating admixture, geography, and temporal variables were used to account for biases in population composition over time. Latitude showed a positive effect on ASD PGS, EA and height, while it had a negative effect on skin pigmentation scores. Additionally, latitude exhibited significant nonlinear effects on multiple phenotypes. The observed patterns highlight the influence of climate-mediated selection pressures on trait evolution. Spline regression revealed that several polygenic scores had nonlinear relationships with years BP. The findings provide evidence for complex evolutionary dynamics, with distinct selective pressures shaping phenotypic diversity across different timescales and environments.

This article argues that a pervasive but confused theory of free will is driving unwarranted resistance to behavioral genetic research and undermining the concept of personal responsibility enshrined in our moral and legal conventions. We call this the theory of ‘free-will-by-subtraction’. A particularly explicit version of this theory has been propounded by the psychologist Eric Turkheimer, who has proposed that human agency can be scientifically quantified as the behavioral variation that remains unexplained after known genetic and environmental causes have been accounted for. This theory motivates resistance to research that suggests genetic differences substantially account for differences in human behavior because that is seen to reduce the scope of human freedom. In academic philosophy, free-will-by-subtraction theory corresponds to a position called ‘libertarian incompatibilism’, which holds that human beings are not responsible for behavior that has extrinsic causes yet maintains that free will nonetheless exists because some fraction of human behavior is self-caused. However, this position is rejected by most professional philosophers. We argue that libertarian incompatibilism is inconsistent with a secular materialist outlook in which all human behavior is understood to have extrinsic causes whether those causes are known to science or not — an outlook Turkheimer shares. We show that Turkheimer sustains this contradiction by adopting an untenable position we call ‘epistemic libertarianism’, which holds that extrinsic causes of our behavior only infringe on our freedom if we know about them. By contrast, the overwhelming majority of secular materialist philosophers support a position called ‘compatibilism’, which maintains that free will is compatible with the comprehensive extrinsic causation of human behavior. We show that compatibilism neutralizes the threat that genetic explanation poses to human agency and rescues a generous conception of personal responsibility that aligns with our moral intuitions.

Incorporating genetic data from diverse populations is crucial for understanding genetic contributions to diseases and ensuring health equity in healthcare practices. However, existing reference panels either capture a limited number of populations or have small sample sizes. We examine the UK Biobank’s performance as a reference for clustering genetically similar individuals. Leveraging data from participants of diverse origins, we aim to improve population representation and mitigate bias caused by the limited number of populations in other reference panels. We combined countries of birth and ethnic backgrounds data fields from the UK Biobank and genetic information to infer genetically similar population labels. A random forest model was then trained on genetic principal components to identify each individual’s most genetically similar population. The model’s performance was validated using the 1000 Genomes and the CARTaGENE biobank data. We identified more diverse reference populations than present in datasets such as 1000 Genomes, covering 19 populations worldwide. Our model achieved medium to high precision and recall for most labeled populations, although lower rates were observed in closely related groups. For instance, we identified 519 people in CARTaGENE most genetically similar to the Middle Eastern reference sample derived in the UK Biobank (there are no Middle Eastern samples in 1000 Genomes), yielding an 81.1% precision and a 97.0% recall rate compared to demographic-based information. This practical approach of clustering genetically similar individuals utilizing existing biobank data may facilitate downstream analyses, such as genomewide association studies or polygenic risk scores in underrepresented populations in genetic studies.

Although financial stressors are implicated as risk factors for suicidal behavior, these associations might be confounded by other factors. Furthermore, a move toward high-risk subgroup definition is necessary. The authors used Swedish national registry data to examine the associations between receipt of social welfare, unemployment benefits, or early retirement (N = 627,745−2,260,753) with suicidal behavior in Cox proportional hazards models. They applied co-relative models to improve causal inference, and examined interactions with aggregate genetic risk for suicidality. All three exposures were associated with elevated suicidal behavior risk. Initial hazard ratios for suicide attempt ranged from 1.37−3.86, were similar for suicide death, and declined after controlling for psychopathology and time elapsed after exposure. Age at registration differentially impacted risk of suicidal behavior. Aggregate genetic liability for suicidality was associated with risk, but its effect was not moderated by financial stress. Financial stressors are associated with suicidal behavior risk even after controlling for psychopathology. Associations are attributable in part to familial confounding, though a potentially causal pathway was observed in most cases. Suicidality risk varied as a function of sex and age at exposure; these findings could be used to identify subgroups at high risk who warrant targeted prevention.

Both peer influences and genetics have been linked to gaming addiction (GA) in adolescents and young adults. This study examined the gene-environment interplay (gene-environment correlation [rGE] and gene-environment interaction [G x E]) between close friends’ gaming (CFG) and GA among South Korean twins. A total of 1462 twins aged 15–29 years (mean = 22.63 ± 2.8 years) completed an online survey that included a 20-item GA measure and a single item assessing CFG. Bivariate Cholesky model-fitting analysis was conducted to examine evidence for rGE in the relationship between GA and CFG. Bivariate G x E model-fitting analysis was performed to determine evidence for G x E effects. A significant genetic correlation (rg = .37; 95% CI [0.24, 0.49]) between GA and CFG supported the role of gene-environment correlation, suggesting that individuals with a genetic predisposition for GA may selectively associate with peers who frequently engage in gaming. The model testing G x E effects indicated that environment-environment interaction was present in the relationship between CFG and GA, such that CFG increased nonshared environmental effects on GA. This pattern provided evidence for peer socialization effects, wherein peers influence the development of GA independently of genetic risk.

Childless individuals have historically faced stigma with assumptions that they lack an interest in future generations because they do not directly contribute to genetic lineage. Individuals share approximately half of their genes with siblings, 12.5% with first cousins, and 6.25% with first cousins’ children. Norwegian census data (2005−2023), reflecting similar trends to the US, UK, and other European countries, indicates a moderate difference in the number of siblings (Parents: 2.03 [women and men]; Childless: 1.88 [women], 1.94 [men]) and nieces/nephews (Parents: 3.99 [women], 4.03 [men]; Childless: 3.32 [women], 3.42 [men]) for 514,777 women and 532,834 men, respectively. By linking four generations through grandmothers, both childless and childbearing women had a slightly higher number of biological extended family members (Parents: 9.63 cousins with 15.79 children; Childless: 8.66 cousins with 12.22 children). Linking four generations for men, numbers were similar: Parents: 9.68 cousins with 15.91 children, Childless: 8.83 cousins with 12.44 children. Based on the average number of children who are parents, the childless have an average genetic fitness that is 49% of that for parents for the next generation. Both parents and childless individuals have a stake in future generations through their biological extended family.

Most twin registries have not systematically collected the data required to determine gender identity, which has limited opportunities to evaluate potential familial contributors to gender diversity. This study addresses this gap by analyzing responses to gender identity questions introduced in Twins Research Australia’s 2023 survey. Among 4475 respondents (mean age 52.2 years, SD = 15.3), 36 (0.8%) indicated a transgender or gender diverse identity, which is consistent with population-based estimates of gender diversity internationally. Gender diversity co-occurred in 2/19 monozygotic pairs and 0/8 dizygotic pairs, giving rise to tetrachoric correlations of 0.62 (95% CI [0.33, 0.87]) and 0.00 (95% CI [0.00, 0.88]), respectively. These results broadly align with previous concordance estimates from twin studies that were specifically focused on gender identity. Although limited by a small sample size, these findings demonstrate the feasibility and utility of systematically collecting gender identity data through routine twin registry surveys.

Research is only beginning to shape our understanding of eating disorders as metabolic-psychiatric illnesses. How eating disorders (EDs) are classified is essential to future research for understanding the etiology of these severe illnesses and both developing and tailoring effective treatments. The gold standard for classification for research and diagnostic purposes has primarily been and continues to be the Diagnostic and Statistical Manual of Mental Disorders (DSM-5). With the reconceptualization of EDs comes new challenges of considering how EDs are classified to reflect clinical reality, prognosis and lived experience. In this article, we explore the DSM-5 method of categorical classification and how it may not accurately represent the fluidity in which EDs present themselves. We discuss alternative methods of conceptualizing EDs, and their relevance and implications for genetic research.

Depression, a leading cause of global disability, arises from a multifaceted combination of genetic and environmental components. This study explores the relationship between major depressive disorder (MDD) polygenic scores (PGS), characteristics and symptoms of depression, and community-shared socioeconomic factors derived from postal code data in a cohort of 12,646 individuals from the Australian Genetics of Depression Study (AGDS). Our findings reveal that people living in areas with relatively higher socioeconomic advantages and education/occupation scores are more likely to report experiencing fewer depressive symptoms during their worst depressive period, as well as fewer number of lifetime episodes. Additionally, participants who reported depression onset later in life tend to currently reside in wealthier areas. Interestingly, no significant interaction between genetic and socioeconomic factors was observed, suggesting their independent contribution to depression outcomes. This research underscores the importance of integrating socioeconomic factors into psychiatric evaluation and care, and points to the critical role of public policy in addressing mental health disparities driven by socioeconomic factors. Future research should aim to further elucidate the causal relationships within these associations and explore the potential for integrated genetic and socioeconomic approaches in mental health interventions.

Prior studies report a decline in male twin live births during economically stressful periods, presumably owing to higher selection in utero against frail male gestations, yet no study has examined the natural corollary: whether provision of economic support increases rates of male twin births. We examined whether male twin live births increase following income gains from the Alaska Permanent Fund Dividend (PFD)—the longest running unconditional cash transfer program in the US. We obtained the monthly volume of male (and female) twin and singleton live births, from January 1980 to December 2019, from Alaska’s Department of Health. Data on PFD timing and payment amounts came from Alaska’s Department of Revenue. We used time-series analyses to gauge whether the odds of male twin live births increase within 2−6 months following PFD receipt, controlling for autocorrelation. Results suggest that for every $1000 increase in PFD payments, the odds of male twin live births increase by 0.002 (p < .05) three months following PFD disbursement. This corresponds with 50 additional (individual) male twin live births statistically attributable to the cumulative PFD amount disbursed over our study period. Income gains through the PFD may correspond with reduced male-specific selection in utero in Alaska.

Despite the decline in mortality rates among children in developing countries, disparities persist between countries, particularly between twins and singletons. This study employed data from nine Demographic and Health Surveys in the Dominican Republic and Haiti to estimate and compare mortality rates for twins and singletons in categories of the under-5 age group (neonatal, postneonatal, and child mortality) and examine the factors associated with excess mortality among twins. From 1996 to 2013, the under-5 mortality rate (U5MR) for singletons in the Dominican Republic declined from 56‰ (95% CI [47, 64) to 30‰ (22–39) and from 108‰ (53–164) to 53‰ (16–89) among twins. In Haiti, between 1994 and 2016, the U5MR declined from 121‰ (109–133) to 77‰ (68–80) for singletons and from 432‰ (327–538) to 204‰ (149–260) among twins. The adjusted risk of neonatal death for twins is 1.4 (1.0–1.9) times higher than for singletons in the Dominican Republic, compared to a risk of 4.3 (3.5–5.3) times higher in Haiti. In the post-neonatal period, the mortality risk for twins in the Dominican Republic was 1.8 (1.0–3.1) times higher than that for singletons, 2.9 (2.3–3.8) in Haiti. The risk of death for twins was not significantly different from that for singletons in both the Dominican Republic and Haiti at ages 1–4 years. Low birth weight, lack of breastfeeding, absence of, or inadequate, antenatal care, noncesarean section birth, and high birth order were associated with excess mortality among twins in both countries.

Is general intelligence (g) a reflective construct, representing a latent causal entity underlying subtest performance, or a formative construct, better understood as an aggregate variable shaped by and summarizing variation across subtests? Genetically informative data provide a framework for testing whether a construct is reflective or formative by comparing common pathway and independent pathways structural equation models (SEMs). Previous studies using biometric SEMs have predominantly supported the reflective model, with phenotypic g mediating the effects of additive genetic and environmental influences on lower level abilities. In the current study, four large genetically informed datasets (three from the US and one from the UK) were analyzed to test three competing SEM models — common pathway, independent pathways, and merged — using Confirmatory Factor Analysis (CFA). Genetic g was estimated in each sample as a latent variable derived from polygenic scores indexing educational attainment and cognitive abilities. The models were compared as follows: the common pathway model, consistent with a reflective g, included a direct path from genetic g to phenotypic g; the independent pathways model, consistent with a formative g, featured indirect paths from genetic g to phenotypic g via subtests; and the merged model incorporated both direct and indirect paths. Across all four datasets, the merged model consistently provided the best fit (based on goodness-of-fit and parsimony criteria). Phenotypic g mediated between 31% and 81% of the effects of genetic g on subtests. These findings suggest that g functions as both a reflective and formative entity.

Sense of humor is a universal human trait, enjoyed daily across cultures. However, little is known about the factors that shape individual differences in humor, particularly what contributes to developing a great sense of humor. While previous studies have identified a significant genetic component for various humor attributes, such as humor appreciation and humor styles, no study has looked at the heritability of humor production ability. This study is the first to assess the genetic and environmental influences on humor production ability using a twin study design. Participants included 448 pairs of monozygotic twins and 196 pairs of dizygotic twins (median age 66 years, mostly female) from the Twins UK registry. Twins self-assessed their humor ability, rated the funniness of their co-twin, and completed an objective humor production task by composing funny captions for captionless cartoons. Additionally, they completed a short cognitive ability test and reported their overall health. Findings revealed that self-rated humor ability was influenced by both additive genetic and nonshared environmental factors. In contrast, objective humor production showed no evidence of additive genetic effects. Instead, all individual differences were shaped by shared and nonshared environmental influences, though a small genetic effect cannot be ruled out. These results suggest that humor production may be more complex and difficult to assess than other cognitive abilities. The study also presents intriguing implications for the evolutionary basis of humor.

Previous observational studies have suggested an association between natural hair color and the risk of endometriosis; however, the causal relationship remains unclear. Here, we conducted a two-sample Mendelian randomization (MR) study to evaluate the potential causal link between natural hair color and endometriosis using 428 single nucleotide polymorphisms (SNPs) as genetic instruments derived from a genomewide meta-analysis comprising over 4511 cases and 227,260 controls of European ancestry. Our findings indicate that dark brown hair is associated with a decreased risk of developing endometriosis (dark brown IVW OR: 0.844, 95% CI [0.725, 0.984], p < .05). Conversely, dark hair color and lighter hair colors (red, blonde, and light brown) did not demonstrate a significant association with endometriosis risk (dark IVW OR: 0.568, 95% CI [0.280, 1.15], p = .117; red IVW OR: 1.058, 95% CI [0.719, 1.558], p = .77; blonde IVW OR: 1.158, 95% CI [0.886, 1.514], p = .28; light brown IVW OR: 1.306, 95% CI [0.978, 1.743], p = .07). These results provide compelling MR evidence supporting a causal association between natural hair color and endometriosis risk. Our findings underscore the need for larger scale studies and randomized controlled trials to delineate the biological mechanisms driving the association between hair color and endometriosis.

The objective of this study was to to describe perinatal outcomes in monochorionic twin pregnancies after selective fetal reduction using bipolar cord coagulation (BCC) or interstitial laser ablation (ILA). This retrospective cohort study included monochorionic twin pregnancies requiring selective fetal reduction between 2008 and 2023 at a referral center in Spain. Maternal and perinatal data were collected and analyzed to compare outcomes between BCC and ILA techniques. The primary outcome was the survival of the co-twin, while secondary outcomes included gestational age at delivery, the incidence of PPROM, birth weight and long-term neurodevelopmental outcomes. Eighty-four procedures were performed (30 ILA, 54 BCC). The overall co-twin survival rate was 80%, with BCC showing a higher survival rate (87%) compared to ILA (67%, p = .026). Fetal death before 24 weeks was more common in ILA (30%) than BCC (7.4%, p = .010). The mean gestational age at delivery was lower in BCC (36.6 weeks) than ILA (38.6 weeks, p = .021), and preterm delivery was more frequent in BCC (50%) compared to ILA (14%, p = .005). BCC seems to have a better overall survival than ILA in complicated monochorionic twins’ selective terminations. However, we could not clarify whether this difference was due to the technique itself or the different gestational age at the time of the procedure as well as the specific indication.

This study aimed to create a risk prediction model with artificial intelligence (AI) to identify patients at higher risk of postpartum hemorrhage using perinatal characteristics that may be associated with later postpartum hemorrhage (PPH) in twin pregnancies that underwent cesarean section. The study was planned as a retrospective cohort study at University Hospital. All twin cesarean deliveries were categorized into two groups: those with and without PPH. Using the perinatal characteristics of the cases, four different machine learning classifiers were created: Logistic regression (LR), support vector machine (SVM), random forest (RF), and multilayer perceptron (MLP). LR, RF, and SVM models were created a second time by including class weights to manage the underlying imbalances in the data. A total of 615 twin pregnancies were included in the study. There were 150 twin pregnancies with PPH and 465 without PPH. Dichorionity, PAS, and placenta previa were significantly higher in the PPH-positive group (p = .045, p = .004, p = .001 respectively). In our model, LR with class weight was the best model with the highest negative predictive value. The AUC in our LR with class weight model was %75.12 with an accuracy of 70.73%, a PPV of 47.92%, and an NPV of 85.33% in our data. Although the application of machine learning to create predictive models using clinical risk factors and our model’s 70% accuracy rate are encouraging, it is not sufficient. Machine learning modeling needs further study and validation before being incorporated into clinical use.

Since the 1970s, twin birth rates have increased sharply in developed countries. In Africa, where the rate is the highest globally, its evolution and variation are poorly understood. This article aims to estimate the twinning rate in sub-Saharan African (SSA) countries throughout 1986–2016 and analyze its spatial, temporal, and ethnic variations. It also seeks to identify social and demographic factors associated with a high probability of twin births and outline a forecast of the twinning rate. We used data from 174 Demographic and Health Surveys from 42 countries. We supplemented them with the UN World Population Prospects (WPP). The twinning rate was calculated by reporting the number of twin births per thousand total births. We used logistic regression to analyze the factors associated with twin births. We projected the twinning rate based on WPP. The overall SSA twinning rate is 17.4 per 1000, but it has changed very little over time, and we expect it will grow a little between 2015 and 2050, increasing at most from 17.4 per 1000 to 18.4 per 1000. We also show significant differences in the twinning rate in SSA according to mother ethnicity. Most ethnic groups with high twinning rates belong to the large Bantu ethnic family. SSA remains the ‘land of twins’, with the twinning rate changing slowly. However, specific health policies must target twin births in SSA to address the public health challenges they present.

Vanishing twin syndrome is a miscarriage of multiples where one or more fetuses ‘vanishes’ (i.e., partial or full resorption or via calcification) during pregnancy, often before detection via ultrasound. It affects 30−50% of multifetal pregnancies, with most cases resulting in full resorption within the first trimester. Despite its recognition since 1945 and rising occurrence in both natural and assisted pregnancies, standardized clinical guidelines remain inadequate, leading to inconsistent diagnosis, counseling, and communication from healthcare providers. This study examines the experiences of mothers and gestational carriers diagnosed with VTS, focusing on patient-provider communication, risks, and symptom disclosure. A global online survey collected qualitative and quantitative data from 153 participants across 17 countries. Results show that most patients with formal diagnoses experienced negative interactions with healthcare providers, with an average sentiment score of −0.7 (on a scale from −2 to 2). Over 53.4% rated their communication experience as −1, and the average satisfaction score for the amount of information received was 3.5/10. Additionally, 43% of respondents were not informed about chorionicity, a key factor affecting fetal outcomes. Significant discrepancies in care were observed across different countries. The findings highlight major gaps in patient-provider communication and inconsistent clinical practices regarding VTS. Addressing these issues through improved education, clearer protocols, and standardized guidelines could enhance patient experiences and decision-making. Future research should focus on provider training and evidence-based strategies to improve the management of VTS and other types of miscarriage and death of multiples during pregnancy and postpartum.

This study aimed to establish normal reference ranges of combined cardiac output (CCO) and umbilical venous flow volume (UVFV) in twin fetuses at 20 to 28 weeks of gestation and to evaluate the differences between monochorionic and dichorionic twins. CCO and UVFV were prospectively measured by ultrasound at two centers. The following exclusion criteria were applied: age <18 years or >45 years, first hospital visit at >16 weeks of gestation, monochorionic monoamniotic twin pregnancy, fetal structural or chromosomal abnormality, fetal growth restriction, twin-twin transfusion syndrome, twin anemia polycythemia sequence, and severe hypertension or renal disease were excluded. The period was divided into three groups: 20–22 weeks of gestation, 23–25 weeks of gestation, and 26–28 weeks of gestation. The CCO and UVFV were measured at least once during each period. CCO and UVFV were collected from 274 and 269 fetuses and were measured 412 and 424 times, respectively. UVFV and CCO levels increased as gestation progressed. The weight-corrected UVFV (UVFV/kg) and CCO (CCO/kg) remained stable. UVFV and CCO did not differ between monochorionic and dichorionic twin fetuses. The mean ± SD of UVFV/kg and CCO/kg were determined as 127.8 ± 31.8 and 439.4 ± 80.1 mL/kg/min, respectively. The UVFV-to-CCO ratio also remained stable from 20 to 28 weeks of gestation, ranging from 27.7% to 31.8%. The values and ranges of UVFV/kg and mean CCO/kg in twins were similar to those in singletons.

The International Society for Twin Studies (ISTS) held its 19th scientific congress in Assisi, Italy, from September 26 to 28, 2024. This prestigious event, which was the seventh joint meeting with the World Congress on Twin Pregnancy, brought together researchers from various fields, including psychology, biology and medicine. Representatives from ICOMBO (the International Consortium of Multiple Birth Organisations), which supports multiple-birth families worldwide, were also in attendance. Many twin researchers consider this event to be the highlight of their professional year, as it brings together experts and parents alike to discuss the latest advancements in twin studies.