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

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          Abstract

          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.

          Highlights

          • Digit ratio (2D:4D) may be a sex-linked correlate of prenatal sex hormone exposure.

          • 2D:4D was examined in a family design study of a general population sample.

          • Correlations for 2D:4D within parent-child dyads were more consistent with X- rather than Y-linkage.

          • There were no significant correlations between 2D:4D of mothers and fathers.

          • 2D:4D correlated positively with age in children, and negatively with age in adults.

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          Most cited references 34

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          2nd to 4th digit ratios, fetal testosterone and estradiol.

          The ratio of 2nd to 4th digit length (2D:4D) is sexually dimorphic (mean 2D:4D is lower in males than females) and is thought to be fixed early in development. 2D:4D has been reported to be related to fetal growth, hand preference, autism, Asperger's syndrome, sperm counts, family size, age at myocardial infarction in men and breast cancer in women. There is indirect evidence that 2D:4D is established in utero and is negatively related to prenatal testosterone and positively with prenatal estradiol. However, there are no studies which show direct relationships between fetal testosterone (FT), fetal estradiol (FE) and 2D:4D. To investigate the relationships between 2D:4D ratios and FT and FE from amniotic fluid. Cohort study. 33 children. Radioimmunoassays of FT and FE obtained from routine amniocentesis; 2D:4D ratios calculated from 2nd and 4th digit length of the right and left hands at age 2 years. A significant negative association between right 2D:4D ratio and FT/FE ratio, which was independent of sex. These preliminary findings lend support to an association between low 2D:4D and high levels of FT relative to FE, and high 2D:4D with low FT relative to FE.
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            The autism-spectrum quotient (AQ): evidence from Asperger syndrome/high-functioning autism, males and females, scientists and mathematicians.

            Currently there are no brief, self-administered instruments for measuring the degree to which an adult with normal intelligence has the traits associated with the autistic spectrum. In this paper, we report on a new instrument to assess this: the Autism-Spectrum Quotient (AQ). Individuals score in the range 0-50. Four groups of subjects were assessed: Group 1: 58 adults with Asperger syndrome (AS) or high-functioning autism (HFA); Group 2: 174 randomly selected controls. Group 3: 840 students in Cambridge University; and Group 4: 16 winners of the UK Mathematics Olympiad. The adults with AS/HFA had a mean AQ score of 35.8 (SD = 6.5), significantly higher than Group 2 controls (M = 16.4, SD = 6.3). 80% of the adults with AS/HFA scored 32+, versus 2% of controls. Among the controls, men scored slightly but significantly higher than women. No women scored extremely highly (AQ score 34+) whereas 4% of men did so. Twice as many men (40%) as women (21%) scored at intermediate levels (AQ score 20+). Among the AS/HFA group, male and female scores did not differ significantly. The students in Cambridge University did not differ from the randomly selected control group, but scientists (including mathematicians) scored significantly higher than both humanities and social sciences students, confirming an earlier study that autistic conditions are associated with scientific skills. Within the sciences, mathematicians scored highest. This was replicated in Group 4, the Mathematics Olympiad winners scoring significantly higher than the male Cambridge humanities students. 6% of the student sample scored 32+ on the AQ. On interview, 11 out of 11 of these met three or more DSM-IV criteria for AS/HFA, and all were studying sciences/mathematics, and 7 of the 11 met threshold on these criteria. Test-retest and interrater reliability of the AQ was good. The AQ is thus a valuable instrument for rapidly quantifying where any given individual is situated on the continuum from autism to normality. Its potential for screening for autism spectrum conditions in adults of normal intelligence remains to be fully explored.
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              Meta-analysis of digit ratio 2D:4D shows greater sex difference in the right hand.

              Our aims are, first, to describe the sex difference in the length ratio of the second and fourth digits (2D:4D), which likely reflects prenatal testosterone levels in humans. Second, to infer the loss of reliability observed in 2D:4D based on self-measured finger lengths. We used random-effects meta-analysis of 2D:4D based on expert-measured finger lengths (116 samples with 13,260 females and 11,789 males). We find a moderate sex difference (with lower 2D:4D for males), which shows substantial heterogeneity (which is unrelated to age). The sex difference is moderated by the type of finger length measurement and by hand. Measurement involving the distortion of soft tissue leads to a significantly larger sex difference than finger length measurement avoiding this. The sex difference in 2D:4D is larger in the right hand than in the left. The reliability of self-measured 2D:4D in the BBC internet study, by far the largest study on 2D:4D, is estimated to be 46% of that of expert-measured 2D:4D. Right-hand 2D:4D might be a better indicator of prenatal androgenisation than left-hand 2D:4D. The view that 2D:4D has allometric properties (Kratochvil L, Flegr J. 2009. Differences in 2nd to 4th digit length ratio in humans reflect shifts along the common allometric line. Biol Lett 5:643-646.) is not supported. Bone lengths contribute to the sex difference in 2D:4D. In addition, there might be a sex difference in fingers' soft tissue, which should be investigated. Because of measurement unreliability, correlations between 2D:4D and variables of interest are about one-third smaller in the BBC internet study than in studies in which 2D:4D is based on expert-measured finger lengths. (c) 2010 Wiley-Liss, Inc.
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                Author and article information

                Contributors
                Journal
                Early Hum Dev
                Early Hum. Dev
                Early Human Development
                Elsevier
                0378-3782
                1872-6232
                1 September 2017
                September 2017
                : 112
                : 14-19
                Affiliations
                [a ]Autism Research Centre, Department of Psychiatry, University of Cambridge, UK
                [b ]School of Social Sciences, Cardiff University, UK
                [c ]MRC Centre for Neuropsychiatric Genetics and Genomics, Division of Psychological Medicine and Clinical Neurosciences and Neuroscience and Mental Health Research Institute, Schools of Psychology and Medicine, Cardiff University, UK
                Author notes
                [* ]Corresponding author at: Autism Research Centre, University of Cambridge, Douglas House, 18b Trumpington Road, Cambridge CB2 8AH, UK.Autism Research CentreUniversity of CambridgeDouglas House, 18b Trumpington RoadCambridgeCB2 8AHUK gvr22@ 123456medschl.cam.ac.uk
                Article
                S0378-3782(17)30251-7
                10.1016/j.earlhumdev.2017.06.006
                5566275
                28668648
                © 2017 The Authors

                This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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