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.

Threat sensitivity, an individual difference construct reflecting variation in responsiveness to threats of various types, predicts physiological reactivity to aversive stimuli and shares heritable variance with anxiety disorders in adults. However, no research has been conducted yet with youth to examine the heritability of threat sensitivity or evaluate the role of genetic versus environmental influences in its relations with mental health problems. The current study addressed this gap by evaluating the psychometric properties of a measure of this construct, the 20-item Trait Fear scale (TF-20), and examining its phenotypic and genotypic correlations with different forms of psychopathology in a sample of 346 twin pairs (121 monozygotic), aged 9–14 years. Analyses revealed high internal consistency and test-retest reliability for the TF-20. Evidence was also found for its convergent and discriminant validity in terms of phenotypic and genotypic correlations with measures of fear-related psychopathology. By contrast, the TF-20’s associations with depressive conditions were largely attributable to environmental influences. Extending prior work with adults, current study findings provide support for threat sensitivity as a genetically-influenced liability for phobic fear disorders in youth.

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.

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.

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.

We evaluated the prevalence and risk factors for child maltreatment in multiples aged 3 years and compared them to singletons in Japanese population-based data. Records on child maltreatment and health check-ups at 3 years of age from 17,125 singletons, 488 twins and 18 triplets were collected from a Public Health Center between April 2007 and March 2011. The associations of child maltreatment with potential risk factors were analyzed using the logistic regression model. Out of all children, 76 (4.31 per 1000) children had documented maltreatment including 69 (4.03 per 1000) singletons and seven (14.31 per 1000) twins. All of the cases in twins were physical abuse (100%) and nearly half of the cases (43%) included emotional abuse. Among twins, 86% of the biological mothers were suspected. The alleged perpetrators of twins showed a significantly higher rate of maternal depression compared to those of singletons. After adjusting the results for a number of potential biological and social risk factors, twins or triplets had a higher risk for maltreatment than singletons (OR 3.39, 95% CI [1.17, 9.83]). Healthcare providers should be aware that a multiple birth can place considerable stress on a family leading to child maltreatment and should provide appropriate support and intervention for mothers with multiples.

Natural selection should favour litter sizes that optimise trade-offs between brood-size and offspring viability. Across the primate order, the modal litter size is one, suggesting a deep history of selection favouring minimal litters in primates. Humans, however – despite having the longest juvenile period and slowest life-history of all primates – still produce twin births at appreciable rates, even though such births are costly. This presents an evolutionary puzzle. Why is twinning still expressed in humans despite its cost? More puzzling still is the discordance between the principal explanations for human twinning and extant empirical data. Such explanations propose that twinning is regulated by phenotypic plasticity in polyovulation, permitting the production of larger sib sets if and when resources are abundant. However, comparative data suggest that twinning rates are actually highest in poorer economies and lowest in richer, more developed economies. We propose that a historical dynamic of gene–culture co-evolution might better explain this geographic patterning. Our explanation distinguishes geminophilous and geminophobic cultural contexts, as those celebrating twins (e.g. through material support) and those hostile to twins (e.g. through sanction of twin-infanticide). Geminophilous institutions, in particular, may buffer the fitness cost associated with twinning, potentially reducing selection pressures against polyovulation. We conclude by synthesising a mathematical and empirical research programme that might test our ideas.