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 antecedent causes yet maintains that free will nonetheless exists because some fraction of human behavioral variation 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 antecedent 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 antecedent 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 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.

We examined how BMI, BMI trajectories, and BMI fluctuation around these trajectories in adolescence were correlated with BMI trajectories and BMI fluctuation in early adulthood, as well as the genetic basis of these associations. BMI data from Finnish twins (N = 1379, 48% males) were collected at ages 11.5, 14, 17.5, 24, and 37 years. BMI trajectories in adolescence (11.5–17.5 years) and early adulthood (17.5–37 years) were estimated using linear mixed-effect models. BMI fluctuation was calculated as the average squared differences between observed and expected BMI around these trajectories. Genetic twin models and a polygenic risk score for BMI (PRSBMI) were used to assess genetic contributions to BMI fluctuation and its associations with BMI and BMI trajectories. Adolescent BMI fluctuation was positively correlated with early adulthood BMI trajectories in females, while in males, adolescent BMI trajectories were positively associated with BMI fluctuation in early adulthood. Genetic factors affected BMI fluctuation in both adolescence and early adulthood when estimated using twin modelling and PRSBMI. Adolescent BMI was positively associated with early adulthood fluctuation in both sexes, with genetic factors playing a role (genetic correlations .08–.29). It was concluded that genetic factors play a significant role in BMI fluctuations in adolescence and early adulthood, with some overlap with the genetics of BMI.

Human eye, skin and hair color pigmentation are highly heritable traits influenced by hundreds of genetic loci. The heritability and genetic etiology of the hyperpigmentation trait pregnancy-related linea nigra (PLN), where a dark but usually temporary vertical line develops on the abdomen, is unknown, and our understanding of its relationships with other pigmentation traits is limited. We conducted a genetic study of self-reported PLN in women of European ancestry, using a genome-based restricted maximum likelihood (GREML) method to estimate PLN heritability, performing a genomewide association study (GWAS) to explore the genetic factors underlying PLN, and calculating polygenic risk scores (PRS) to assess whether this trait shares genetic liability with two other skin pigmentation phenotypes, skin colour and mole count. We found 35% of the variance in developing PLN was explained by common genetic variation. The GWAS revealed four genomic loci suggestively associated (p values ≤ 1 × 10-6) with PLN: rs1263154 near the UPP2 gene (p = 9.0 × 10-7), rs26331 near SEMA6A (p = 6.6 × 10-7), rs78371540 in OLFM3 (p = 5.5 × 10-7), and rs72693263 near FLRT2 (p = 1.1 × 10-7). Of these genes only OLFM3 has been previously associated with pigmentation. Our PRS results provide the first evidence that genetic factors underlying skin color and mole count also contribute to the development of PLN in women of European ancestry.

Twin fetuses show a physiologically slower growth rate in the third trimester compared to singletons. Therefore, it has been suggested that the anthropometric evaluation of twins at birth should be performed using twin-specific charts. To be reliable, anthropometric charts need to fulfil certain methodological criteria to systematically review studies that have developed neonatal twin-specific anthropometric charts and assess their methodological quality. A comprehensive search was conducted across Cinahl, Embase, Global Index Medicus, PubMed, Scopus, Web of Science, Google Scholar, and relevant preprint repositories, followed by a predefined snowballing search. Two reviewers independently screened studies for eligibility, selecting those that have developed cross-sectional anthropometric charts for twins at birth, published since January 1990. Articles were excluded if they did not use anthropometric measurements at birth to construct charts. Two reviewers independently extracted data and performed quality assessment with a 16-item grading system. Sixty-eight studies were included, with all but one constructed reference, not standard, charts. Most studies did not meet the quality criteria: only 9% were prospectively designed, 34% reliably estimated gestational age, 19% reported standardized instruments, 28% described measurement techniques, and 26% did not stratify centiles by sex. This review reveals considerable methodological limitations in existing twin-specific neonatal anthropometric charts.

Igbo-Ora, a town in southwestern Nigeria, is renowned for exceptionally high dizygotic twin birth rates, recording approximately 45 per 1000 live births. This article explores the factors behind this unique phenomenon by critiquing the community’s perceptions and narrative of the factors responsible for the high twinning rate and comparing these perceptions with biomedical hypotheses. Drawing on 6 months of ethnographic fieldwork — participant observation, 81 semistructured interviews, and FGDs — this study documents local narratives that highlight hereditary ‘twin threads’ —; specific foods, notably Ilasa (okra-leaf soup) and cassava meals; environmental qualities of ‘air’ and ‘water’; and divine sanction as factors responsible for the incidence of twin birth in Igbo-Ora. These local narratives are analyzed against certain biomedical perspectives on maternal age and parity effects, putative genetic variants influencing gonadotrophins, and dietary phytoestrogens. The study found that the community resist single-cause explanations for the incidence of twin birth and instead articulates a complementarity of genetic, ecological, dietary, and spiritual factors. This holistic framing contrasts with and complements prevailing genetic and nutritional theories surrounding the incidence of twin birth. The article argues that future genetic and epidemiological investigations in high-twinning populations must be culturally attuned to ensure accurate phenotype definition, ethical engagement, and translational relevance.

The most accurate confidence intervals for estimates of heritability are based on Fisher’s Z-transformation. Using these methods, Loehlin and Nichol’s (1976) analysis is confirmed, viz., that it is pointless to estimate heritabilities from the classical twin method unless one is prepared to recruit upwards of 800 twin pairs for study. Even then the assumptions of that method are so improbable as to leave reasonable doubt about the true value of H. Estimates of H from the correlation of monozygotic twins reared apart (MZA twins), on the other hand, is remarkably more stable than that obtained by comparing MZ with DZ correlations. Moreover, estimates of H based on the MZA design rest upon more reasonable (and often testable) assumptions.

Triplet pregnancies are linked to a higher risk of maternal and fetal complications compared to twins and singletons. Chorionicity has been suggested as a key factor influencing perinatal and maternal outcomes in triplet pregnancies; however, more evidence is needed to clarify its role. This study aimed to assess the impact of chorionicity on adverse maternal, fetal, and neonatal outcomes in triplet pregnancies. This retrospective observational study was conducted on triplet pregnancies delivered between 2010 and 2025 at Florence University Hospital, a tertiary referral maternity center. A total of 77 triplet pregnancies were included, 51 trichorionic and 26 nontrichorionic, resulting in the delivery of 214 newborns. Maternal characteristics and obstetric, fetal, and neonatal complications were compared based on chorionicity. Multivariate logistic regression analyses identified complications most strongly associated with chorionicity, after adjusting for maternal age, mode of conception, and gestational age at delivery. Women with nontrichorionic triplet pregnancies had higher risks of delivering earlier and postpartum hemorrhage. Their newborns had significantly lower birth weights, a greater need for neonatal intensive care admission and respiratory support, and higher rates of hypoglycemia and sepsis. The multivariate logistic regression confirmed nontrichorionicity as an independent factor associated with earlier delivery and postpartum hemorrhage. Chorionicity plays an essential role in determining the prognosis of triplet pregnancies. Nontrichorionic pregnancies are strongly linked to earlier delivery, lower birth weights, and increased neonatal complications. These findings are useful for counseling patients, helping them understand specific risks based on chorionicity, although intensive prenatal care remains critical for all triplet pregnancies.

A pair of dizygotic (DZ) twins discordant for Turner syndrome are discussed with reference to the biological origins of the condition and the effects of discordance on the twin relationship. There is little research on how having an atypical twin influences the life events and goals of the typical twin. Next, timely reviews of research on preventing premature twin birth, a twin gestation with hydatidiform mole, an update on Feingold syndrome twins discussed in a previous issue of this journal, and qualitative monozygotic twin difference studies are presented. The final portion of this article covers human interest stories of twins that are variously entertaining and enlightening. They include identical twins who celebrated their 100th birthday together, twins in famous families, celebration of the Yorùbá twins of Nigeria, identical artistic partners, and surgical separation of a rare, conjoined twin set.