Testing the extreme male brain theory of autism spectrum disorder in a familial design

Second-to-Fourth Digit Ratio (2D:4D) in Psychiatric Disorders: A Systematic Review of Case-control Studies

The second-to-fourth digit ratio (2D:4D) is an indirect, retrospective, non-invasive measure that correlates negatively with intrauterine exposure to testosterone. The present meta-analysis aimed to evaluate if 2D:4D differs between patients with psychiatric disorders and controls. In September 2019, we searched in Web of Knowledge, PsycINFO, Embase, and CINHAL, and retrieved 619 papers. We finally included 43 case-control studies which compared the 2D:4D ratio of patients with autism spectrum disorder (ASD) (n = 16), schizophrenia (n = 8), gender non-conformity (n = 7), addictions (n = 5), attention deficit-hyperactivity disorder (ADHD) (n = 4), mood disorders (n = 2), and intellectual disability (n = 1) to non-clinical controls. Meta-analyses showed that, overall, psychiatric patients had lower 2D:4D than healthy controls (n = 43, overall sample = 9,484, mean difference = -0.0056, 95% confidence interval from -0.0093 to -0.002, I2 = 74%), with more pronounced differences in the right hand, males, and children. Considering psychiatric disorders individually, significant differences were found in the ASD, ADHD, and addictions groups, in which 2D:4D was significantly lower than healthy controls. Conversely, the right hand of males with schizophrenia showed higher 2D:4D than healthy controls. No other significant differences were detected. Although our results need to be cautiously interpreted and find limited applications in clinical practice, they may suggest that 2D:4D is altered in some psychopathological conditions, underlining the role of prenatal exposure to sex steroids in the etiology of psychiatric disorders.

Assortative mating and digit ratio (2D:4D): A pre-registered empirical study and meta-analysis

BACKGROUND

It has been hypothesised that the ratio of length between the second and fourth fingers (2D:4D), commonly employed as an indicator of foetal sex hormone exposure, may be positively correlated between heterosexual partners.

AIMS

As previous evidence has been conflicting, our study aimed to determine whether intra-couple correlations exist for digit ratio variables, and if so, to estimate the size and direction of these effects.

STUDY DESIGN

We present a preregistered (osf.io/6jg8p) correlational study and quantitative meta-analysis of the available literature, and attempted to locate further published and unpublished data (i.e. 'grey literature') by contacting n = 248 researchers in the 2D:4D and related fields.

SUBJECTS

n = 58 heterosexual dating couples from the UK took part in our empirical study, and the meta-analysis included data from k = 11 samples.

OUTCOME MEASURES

We measured digit ratio for the right hand (R2D:4D), left hand (L2D:4D), and average of both hands (M2D:4D), as well as the right-left-difference (D_[R-L]).

RESULTS

We found no evidence of significant positive intra-couple correlations in our own data, but a significant (positive) meta-analytic effect size estimate emerged for R2D:4D (r = 0.072, p = 0.014). The meta-analytic effects for L2D:4D (r = 0.043, p = 0.303), M2D:4D (r = 0.070, p = 0.225), and D_[R-L] (r = 0.028, p = 0.649) were all in the same direction but not statistically significant. However, if the sample from Klimek et al. (2014, 2016) were omitted, meta-analysis would also yield a significant positive correlation for M2D:4D (r = 0.128, p = 0.001).

CONCLUSIONS

Although our findings are based on a fairly small range of studies, which themselves provide a relatively small sample of participants, they do imply the intriguing possibility of small effects of positive assortment in relation to characteristics associated with the prenatal hormonal environment.

Maternal polycystic ovarian syndrome in autism spectrum disorder: a systematic review and meta-analysis

There is evidence showing a positive correlation between prenatal androgens and their effect on the development of central nervous system and the autistic spectrum disorder (ASD) phenotype in offspring of mothers with polycystic ovary syndrome (PCOS). We applied a systematic review to investigate whether women with PCOS have increased odds of having a child with ASD, while, secondarily, if these women themselves are at high risk of having the disease. Major databases from inception until 14th October 2018 were searched. The primary outcome measure was the odds of an ASD diagnosis in children of mothers with diagnosed PCOS, while the secondary outcome was the odds of ASD diagnosis in women with PCOS. Scheduled subgroup analyses were according to the time of birth and maternal age. We assessed the odds ratio (OR), using a random-effects model; heterogeneity was assessed by I² and τ² statistics. The quality of the evidence was evaluated using the Newcastle-Ottawa Scale. Ten studies were eligible for inclusion, including a total of 33,887 ASD children and 321,661 non-ASD children. Diagnosed PCOS was associated with a 1.66 times increase in the odds of ASD in the offspring [95% CI: 1.51, 1.83, p = 1.99 × 10^-25, 7 studies, I² = 0%, τ² = 0]. Women with PCOS were 1.78 times more likely to be diagnosed with ASD (95% CI: 1.10, 2.87, p = 0.0179, 5 studies, I² = 85.4%, τ² = 0.2432). Additional analyses did not change the initial result. The overall quality of the evidence was high. The pooled effects size displayed low heterogeneity (I² = 0%) for the primary outcome. While the heterogeneity in the secondary outcome appears to attenuate when only high quality studies are synthesized, still the result exhibits significant heterogeneity. Τhe available data allowed a subgroup analysis only for classification system for PCOS diagnosis and showed a significant increase of ASD diagnosis in the offspring of women with Read Code and ICD diagnosed PCOS. In conclusion, the available evidence suggests that women with PCOS have increased odds of having a child with ASD, an effect size estimate based on a large number of patients from studies of good quality. Regarding the evidence on the prevalence of ASD in PCOS women, results suggest that women with PCOS are more likely to be diagnosed with ASD.

The 2D:4D ratio is associated with performance in the 'TEACCH program' of subjects with autism spectrum disorder

Problems arising on the autistic spectrum are more common in male individuals, leading scholars to attempt to determine whether there is a correlation with testosterone levels to which fetuses were exposed during intrauterine life. The aim of the present study was to investigate, through the digit ratio technique, the possible correlation between testosterone exposure during intrauterine life and the achievement of predetermined objectives during the educational program Treatment and Education of Autistic and Related Communication Handicapped Children (TEACCH) in a group of subjects with different forms of autism. The results showed a positive correlation of the TEACCH program and 2D:4D ratio. Therefore, we hypothesize that a base screening may be useful to pinpoint the optimal teaching strategies to obtain the best possible performance from each subject.

Familial digit ratio (2D:4D) associations in a general population sample from Wales

BACKGROUND

The relative length of the second and fourth fingers (2D:4D) may be a sex-linked correlate of prenatal androgen exposure. However, the nature of the sex-linkage is controversial, with evidence for both X- and Y-linkage of the 2D:4D phenotype.

AIMS

To examine transgenerational effects relating to sex-linkage. In addition, assortative mating on 2D:4D was considered, as well as associations between 2D:4D and age and sex.

STUDY DESIGN

A family study was conducted. Parents and offspring completed a demographic questionnaire, and digit ratios were calculated from photocopies of participants' hands.

SUBJECTS

We recruited and phenotyped 585 individuals attending a cultural festival in Wales. 2D:4D information was available for 47 mother-son dyads, 70 mother-daughter dyads, 31 father-son dyads and 30 father-daughter dyads.

OUTCOME MEASURES

Correlations between 2D:4D of parents and children, as well as between mothers and fathers were conducted. 2D:4D was also examined in relation to age and sex.

RESULTS AND CONCLUSIONS

There was a sex difference in 2D:4D (males<females). Within the dyads, there was a significant positive correlation between mother and daughter 2D:4D, but no significant correlation between mother and son ratios, nor between father and offspring ratios. The overall pattern of correlations (with emphasis on father-son dyads) was not supportive of Y-linkage. There was a positive correlation between 2D:4D and age in children, and a negative correlation between 2D:4D and age in adults, and no evidence of assortative mating. Our data are consistent with the notion of 2D:4D as a sexually-dimorphic, mildly age-sensitive, and transgenerationally-transmitted trait that is more likely to be X- than Y-linked.

The Use of Testosterone/Cortisol Ratio in Response to Acute Stress as an Indicator of Propensity to Anger in Informal Caregivers

Caring for an offspring diagnosed with a psychological chronic disorder is used in research as a model of chronic stress. Indeed, it is usually associated with disturbances in the salivary cortisol (Csal) levels of the caregiver. An imbalance between salivary testosterone (Tsal) and Csal levels is a marker of proneness to social aggression. Given this, we aimed to establish whether the salivary testosterone/cortisol (Tsal/Csal) ratio response to acute stress could be employed as a marker of proneness to anger in informal caregivers of offspring with autism spectrum (ASD). Tsal/Csal ratio and anger responses to a set of different cognitive tasks as well as anger trait and expression were compared in these informal caregivers and controls. Caregivers, particularly those of offspring with ASD, had higher Tsal/Csal ratios than controls in response to acute stress, concretely after the stress in the case of fathers (p = .05) and before stress when analyzing mothers (p = .05). Moreover, ASD fathers and mothers obtained higher magnitude of the T/C ratio response to stress (p = .03 and p =.04, respectively), anger state (p = .02 and p = .02, respectively) and expression scores (p = .05 and p = .05, respectively) than controls. Finally, high Tsal/Csal ratio levels and response to stress were significantly associated with high anger feelings increases (p .05, respectively) and expression (p .05, respectively) in caregivers.

Language Impairments in ASD Resulting from a Failed Domestication of the Human Brain

Autism spectrum disorders (ASD) are pervasive neurodevelopmental disorders entailing social and cognitive deficits, including marked problems with language. Numerous genes have been associated with ASD, but it is unclear how language deficits arise from gene mutation or dysregulation. It is also unclear why ASD shows such high prevalence within human populations. Interestingly, the emergence of a modern faculty of language has been hypothesized to be linked to changes in the human brain/skull, but also to the process of self-domestication of the human species. It is our intention to show that people with ASD exhibit less marked domesticated traits at the morphological, physiological, and behavioral levels. We also discuss many ASD candidates represented among the genes known to be involved in the “domestication syndrome” (the constellation of traits exhibited by domesticated mammals, which seemingly results from the hypofunction of the neural crest) and among the set of genes involved in language function closely connected to them. Moreover, many of these genes show altered expression profiles in the brain of autists. In addition, some candidates for domestication and language-readiness show the same expression profile in people with ASD and chimps in different brain areas involved in language processing. Similarities regarding the brain oscillatory behavior of these areas can be expected too. We conclude that ASD may represent an abnormal ontogenetic itinerary for the human faculty of language resulting in part from changes in genes important for the “domestication syndrome” and, ultimately, from the normal functioning of the neural crest.

Digit ratio and autism spectrum disorders in the Avon Longitudinal Study of Parents and Children: a birth cohort study

OBJECTIVES

To investigate whether second-to-fourth digit ratio (2D:4D), a measure commonly used as a proxy for fetal testosterone exposure, is associated with autism spectrum disorders (ASDs), as predicted by the extreme male brain theory of autism.

DESIGN

A birth cohort study.

SETTING

The Avon Longitudinal Study of Parents and Children (ALSPAC).

PARTICIPANTS

6015 ALSPAC children with data on digit ratio, at least 1 outcome measure and information on potential confounding variables (parental occupational class, maternal education and age at digit ratio measurement). Digit ratio was measured by the photocopy and calliper method.

OUTCOMES

ASD diagnosis (cases were identified previously by record linkage or maternal report) and 4 measures that combine optimally within ALSPAC to predict ASD: the Children's Communication Checklist (coherence subscale), the Social and Communication Disorders Checklist, a repetitive behaviour measure, and the Emotionality, Activity and Sociability scale (sociability subscale). These measures were dichotomised, with approximately 10% defined as the 'risk' group.

RESULTS

Using logistic regression, we examined the association of 2D:4D with ASDs and 4 dichotomised ASD traits. Covariates were occupational class, maternal education and age at 2D:4D measurement. 2D:4D was not associated with ASDs in males (adjusted OR per 1 SD increase in mean 2D:4D, 0.88 (95% CI 0.65 to 1.21), p=0.435) or females (adjusted OR=1.36 (95% CI 0.81 to 2.28), p=0.245). Similar results were observed after adjustment for IQ. There was 1 weak association between reduced coherence and increased left 2D:4D in males, in the opposite direction to that predicted by the extreme male brain theory (adjusted OR=1.15 (95% CI 1.02 to 1.29), p=0.023). Given multiple comparisons, this is consistent with chance.

CONCLUSIONS

In this population-based study, there was no strong evidence of an association between 2D:4D and ASD diagnosis or traits, although the CIs were wide. These results are not consistent with the extreme male brain theory.

Sex-different abnormalities in the right second to fourth digit ratio in Japanese individuals with autism spectrum disorders

Background

The prevalence of autism spectrum disorders (ASDs) is higher in men than in women. The extreme male brain theory proposes that excessive prenatal testosterone activity could be a risk factor for ASDs. However, it is unclear whether prenatal sex hormone activity is a risk factor for women. The ratio of the length of the second to fourth digits (2D:4D) is considered to be a biomarker of the prenatal ratio of testosterone to estrogen. Therefore, this study compared the 2D:4D ratios of women with and without ASDs to determine if prenatal sex hormone activity could be a risk factor for ASDs in women.

Methods

The study included 35 Japanese men with ASDs, 17 Japanese women with ASDs, 59 typically developed (TD) Japanese men, and 57 TD Japanese women. We measured digit lengths and compared the 2D:4D ratios among the four groups. We also examined the relationship between the 2D:4D ratio and the autism-spectrum quotient score of each group.

Results

In our cohort, men with ASDs tended to have lower right-hand 2D:4D ratios relative to TD men. In contrast, the right 2D:4D ratios in women with ASDs were higher compared to those of TD women. No significant correlations were found between the 2D:4D ratios and the autism-spectrum quotient scores in any group. The higher right 2D:4D ratios in women could not be explained by age or full-scale intelligent quotients. This group difference was not found for the left 2D:4D or right–left 2D:4D ratios.

Conclusions

We found a reverse direction of abnormality in the right 2D:4D ratio for men and women with ASDs. It has been posited that high prenatal testosterone levels lead to a lower 2D:4D ratio. However, a recent animal study showed that testosterone injection to dam leads to a higher right 2D:4D ratio especially for female offspring, which might be mediated by abnormal adipose accumulation in the fingertip. Therefore, the present findings suggest that high prenatal testosterone could be a risk factor both for Japanese men and women with ASDs, elucidating one potential etiology of ASDs in women.