The author presents a commentary on the trilogy of papers by Damien Morris entitled ‘Behavioral Genetics and Human Agency: How Selectively Deterministic Theories of Free Will Drive Unwarranted Opposition to Behavioral Genetic Research and Undermine Our Moral and Legal Conventions’, published recently in this journal [PMID 40457806, 40963213, 41107251]. In addition to commenting on this specific set of papers, the author offers a broader perspective and defense of the field of behavioral genetics.

Myopia is an increasing global health concern and a leading cause of visual impairment. Genetic factors play a major role, and polygenic risk scores (PRSs) may help identify children at high risk of developing myopia. However, most PRSs are based on European populations, and accurately predicting risk across ancestries remains a challenge. We developed and evaluated PRSs for spherical equivalent refractive error (SER) and myopia using multitrait and multi‑ancestry genomewide association study data. A multitrait analysis of SER‑correlated traits identified 709 genomewide significant loci. PRSs were generated with SBayesRC for each ancestry group and for a combined multi‑ancestry model, and validated in the Australian Twins Eye Study and non‑European participants from the UK Biobank. The European PRSs explained approximately 20% of SER variance in Europeans and 18% in admixed Europeans and showed good transferability to South Asian (14%), East Asian (13%), and African (8%) groups. A multi‑ancestry PRS further improved prediction in Africans, explaining 9% of the variance. Predictive accuracy for high myopia was strong in the admixed group (AUC = 0.82, 95% CI [0.78, 0.87]), with all ancestry groups achieving AUCs of at least 0.70; European ancestry data were not available. PRS also predicted axial length in children, particularly those aged 5–8 years, where individuals in the lowest 10% of the PRS distribution had significantly longer axial lengths (β = 0.81 mm, p = 5.71 × 10−3). These findings enhance genetic prediction of SER and myopia, showing the potential of multitrait, multi-ancestry PRS for early, equitable risk stratification.

Observational studies report a correlation between height and intelligence, but causality and mechanisms remain unclear due to confounding and reverse causation. Using large-scale genome-wide association study (GWAS) summary data, we performed bidirectional two-sample Mendelian randomization (MR) analyses and two-step mediation analyses to examine the associations between height and intelligence (general and fluid), and potential mediation by neuroticism, body mass index (BMI), anxiety, and major depression. Genetically predicted height was positively associated with intelligence (general intelligence: β = 0.115, p = 1.2 × 10−19; fluid intelligence: β = 0.266, p = 5.7 × 10−22). Height was negatively associated with neuroticism (β = −0.032, p = 7.0 × 10−5) and BMI (β = −0.067, p = .002), which were in turn negatively associated intelligence. Mediation analyses showed significant indirect effect of neuroticism (6.09%, p = 2.6 × 10−4) and BMI (7.83%, p = 6.0 × 10−3). No mediation was found for anxiety or depression. Reverse MR also suggested positive associations of intelligence with height. This study provides genetic evidence for bidirectional association between height and intelligence, with neuroticism and BMI as potential mediators. These findings improve our understanding of the height-intelligence relationship and its underlying psychological and metabolic pathways.

Hyperuricemia is a common metabolic disorder and has become a global health concern. This study investigated the association between DNA methylation (DNAm) and serum uric acid (SUA) by conducting an epigenomewide association study (EWAS) in Chinese monozygotic (MZ) twins. Genomewide DNAm of 50 MZ twin pairs was profiled using the Infinium MethylationEPIC v2.0 BeadChip (935K). Generalized estimating equations (GEE) were used to examine the association between DNAm and SUA. Causal relationships between DNAm and SUA were assessed using ICE FALCON approach. Associations between mRNA expression and SUA were further assessed. Finally, candidate genes identified through epigenomewide association study (EWAS), causal inference, and gene expression analyses were validated in a longitudinal twin study. We identified 70 CpGs, mapping to genes such as DOK6 and NGLY1, significantly associated with SUA (Bonferroni correction p < 5.8 × 10−8). Causal analyses revealed one CpG with a causal effect of DNAm on SUA, 22 CpGs with causal effects of SUA on DNAm, and 33 CpGs showing bidirectional causality. Eleven genes displayed expression levels associated with SUA. DOK6, NGLY1, PKM, and SLC44A1 were selected as candidate genes, all of which showed unidirectional causal effect of SUA on DNAm. In the longitudinal analysis, baseline SUA levels (2012–13) were associated with subsequent DNAm levels in DOK6 and NGLY1 genes (2023–24). In conclusion, we found that SUA levels may influence DNAm variations, particularly at CpG loci within the DOK6 and NGLY1 genes. These findings provide key clues for future investigations into the mechanisms linking SUA with its epigenetic regulatory pathways.

This study examines birth rates and infant mortality rates in Brazil among singletons, twins, and higher order multiples between 2010 and 2023. Data were obtained from the Brazilian Ministry of Health’s Live Birth Information System and Infant Mortality Information System. The dataset comprised 39,663,928 live births and 464,843 infant deaths occurring within the first year of life. Twin birth rates increased steadily over time, while singleton birth rates declined. The highest twin birth rates were observed in the Southeast (11.64‰) and South (11.47‰), whereas the lowest were observed in the North (7.81‰) and Northeast (9.23‰). Infant mortality among twins was approximately five times higher than among singleton, and higher order multiples faced an approximately threefold greater risk compared to singletons. Infant mortality rates were highest in the North and Northeast and lowest in the Southeast and South. Sex ratios varied by gestation type, with singletons showing the expected male bias and twins a slightly reduced male proportion, reflecting biological and regional influences on birth rates in Brazil. The data reveal a marked demographic shift between 2010 and 2023, with birth rates declining among women in their teens and 20s and increasing among those in their late 30s and 40s. Using Weinberg’s differential method, we identified divergent trends in zygosity: dizygotic birth rates increased significantly over time, whereas monozygotic rates remained stable. This contrast underscores the environmentally responsive nature of dizygotic twinning, particularly its association with advancing maternal age and assisted reproductive technologies, compared to the relatively biologically stable pattern of monozygotic twinning.

Twin studies provide a powerful framework to identify genetic and environmental influences on health and development, particularly during infancy, childhood, and adolescence. Although twin research has expanded globally, evidence from South Asia remains limited and poorly characterized. This systematic review aimed to synthesize health research involving infant, child, and adolescent twins conducted in South Asia and to identify methodological patterns and research gaps relevant to future twin research in the region. A systematic review was conducted according to a registered protocol (PROSPERO: CRD42018105704) using the PICOS framework. Multiple international and regional databases were searched from inception to September 2025. Studies recruiting twins aged 0–19 years within the South Asian Association for Regional Cooperation (SAARC) countries were included. Study selection, data extraction, and quality appraisal were conducted independently, with study quality assessed using Joanna Briggs Institute critical appraisal tools. Results were synthesized narratively due to heterogeneity across study designs and outcomes. Eighteen studies were included from 1245 identified records. Most studies focused on twin pregnancy and neonatal outcomes (61%), followed by dentistry, ocular health, and gut microbiota. Twin pregnancies were consistently associated with increased risks of low birth weight, preterm birth, and perinatal morbidity and mortality. A limited number of studies applied classical twin designs to investigate genetic and environmental contributions. Twin research involving infants, children, and adolescents in South Asia remains limited in scope and methodological depth. Strengthening twin registries, methodological standardization, and collaborations is essential to advance genetically informative research and enhance the global representativeness of twin studies.

Self-concept in adolescence plays a key role in psychological adjustment, yet its development in twins remains underexplored — especially in Central Europe. While previous studies suggest minor differences between monozygotic (MZ) and dizygotic (DZ) twins, cultural and contextual influences may affect how twins perceive themselves. This mixed-method study examined 336 Czech adolescent twins (168 twin pairs; 32 MZ, 136 DZ) aged 11–15 years. Participants completed three standardized instruments: the Behavioral and Psychological Adjustment Questionnaire (BPQA), the School Performance and Adjustment Scale (SPAS), and the Rosenberg Self-Esteem Scale (RSES). Quantitative analyses included t tests, Analyses of Variance (ANOVAs), and Pearson correlations. To triangulate findings, focus group interviews were conducted with three twin pairs and analyzed thematically. No statistically significant differences were found between MZ and DZ twins across BPQA, SPAS, and RSES measures. However, small, nonsignificant trends suggested slightly higher emotional self-concept scores in MZ twins. A modest gender difference emerged: girls reported more internalizing symptoms. Correlational analysis showed that lower behavioral difficulties were associated with higher school adjustment and self-esteem. Thematic analysis from focus groups confirmed key patterns such as the emotional safety of twinship and the struggle for individuality. Zygosity does not appear to be a major predictor of self-concept in early adolescence. Instead, self-perception in twins is shaped by a complex interplay of mental health, family dynamics, and peer context. Future research should compare twin self-concept development across cultures to further examine sociocultural influences.

Twin research informs our thinking about possible human reproductive cloning (HRC). Other human models for assessing the benefits and difficulties of HRC are look-alike parent-child and sibling pairs. An in-depth interview with an extraordinary mother-daughter pair is provided. This section is followed by reviews of research on infantile pyknocytosis in a dizygotic (DZ) twin, monozygotic (MZ) twin discordance for hemimicrencephaly, unusual dental development in a twin, and ‘gaze fingerprint signatures’. The final section presents human interest stories of identical twin executives at odds, the loss of identical twin Jim Whittaker, identical twin artists, and twin wisdom captured in stone.