Digit Ratio: A Pointer to Fertility, Behavior, and Health

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Could the length of your fingers indicate a predisposition to breast cancer? Or musical genius? Or homosexuality? In Digit Ratio, John T. Manning posits that relative lengths of the second and fourth digits in humans (2D:4D ratio) does provide such a window into hormone- and sex-related traits.

It has been known for more than a century that men and women tend to differ in the relative lengths of their index (2D) and ring (4D) fingers, which upon casual observation seem fairly symmetrical. Men on average have fourth digits longer than their second digits, while women typically have the opposite. Digit ratios are unique in that they are fixed before birth, while other sexually dimorphic variables are fixed after puberty, and the same genes that control for finger length also control the development of the sex organs. The 2D:4D ratio is the only prenatal sexually dimorphic trait that measurably explains conditions linking testosterone, estrogen, and human development; the study of the ratio broadens our view of human ability, talent, behavior, disposition, health, and fertility. In this book, Manning presents evidence for how 2D:4D correlates with traits ranging from sperm counts, family size, musical genius, and sporting prowess, to autism, depression, homosexuality, heart attacks, and breast cancer, traits that are all linked with early exposure to sex hormones.

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From The Critics
Manning (biological sciences, University of Liverpool) argues that the relative lengths of the second and fourth fingers are genetically linked to hormone- and sex-related traits, and by extension with sperm counts, family size, musical genius, sporting prowess, autism, depression, homosexuality, heart problems, and breast cancer. He suggests that the study of the ratio between the lengths of the second and fourth finger can broaden our understanding of human ability, behavior, and health. Three photographs, 38 figures, and eight tables are included. Annotation c. Book News, Inc., Portland, OR (booknews.com)
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Product Details

  • ISBN-13: 9780813530307
  • Publisher: Rutgers University Press
  • Publication date: 1/28/2002
  • Series: Series in Human Evolution
  • Edition description: New Edition
  • Pages: 192
  • Product dimensions: 6.10 (w) x 9.20 (h) x 0.50 (d)

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Excerpt from Digit Ratio: A Pointer to Fertility, Behavior, and Health by John T. Manning

Copyright information: http://rutgerspress.rutgers.edu/press_copyright_and_disclaimer/default.html
At first sight the essence of the uterine environment appears to be protection. The fetus differentiates and grows, apparently isolated from the harsh Darwinian world experienced by children and adults. However, this impression of an idyll with its watery, membranous peace is misleading. It is essentially the result of inaccessibility. Common-sense observation can make little of prenatal differentiation and the scientist, bound about by ethical considerations, is often forced to make do with animal models. If there were ready access to prenatal development we would most likely see evidence for gene action which carries with it the familiar balance of advantage and disadvantage. Natural selection removes harmful genes from populations, but the power of selection is blunted when a gene has good and bad influences. This is likely to be true in the uterine environment, and variations in prenatal development may well underlie the variations in the behavior, fertility, and health of children and adults. The genes involved may give some individuals a bad start in life or predispose them to ill health or infertility in later life. In others they may confer robust physical and mental health and fertility. In order to understand the prenatal causes of ill health and why such genes are maintained in the population we require a "window" into uterine conditions. By necessity the window has to be a developmental record which has been fixed during prenatal life, preferably towardthe end of the first trimester, when important organ systems are still being formed. My thesis in this book is that the relative lengths of the 2nd and 4th digits (2D:4D) provides a developmental window which speaks to us about sex-related behavior, fertility, and health. If this is so the ratio will be important in our understanding of patterns of fecundity, behavior, illness, and even differences between human populations.
But first (a) how are the 2nd and 4th digits measured; (b) how does the ratio differ between sexes and when in ontogeny does the difference arise; and (c) how does the 2D:4D ratio differ between species and human populations?
Look at the hands in figure 1.1. Hand A is that of a man, and B is a woman's hand. The 2nd or index finger and the 4th or ring finger present an appearance of symmetry on either side of the long 3rd or middle finger. However, if one measures from the basal crease of the digits to the tip you will find that in the male hand the length of the index finger is 92% that of the ring finger. In other words, the ratio of 2nd:4th is 0.92. The female hand has a 2nd digit that is approximately the same length as the 4th digit. It's 2D:4D ratio is therefore 1.00.
Can the ratio be measured accurately? Consider data obtained from the 2nd and 4th digits of 300 hands from as many subjects. Each digit was measured twice using vernier calipers measuring to 0.01 mm. Using repeated measures ANOVA analysis we can compare the error mean squares (i.e., the differences between the repeated measurements) and the groups mean squares (i.e., the differences between the subjects). The ratio between the two (the F ratio) can then be tested to determine whether the differences between subjects are significantly larger than the errors in measurement. Considering the 2nd digit we find that F = 341.81 and p = < 0.0001. This means that differences between subjects are more than 340 times greater than measurement error. A similarly large F value was found for the 4th digit (F = 375.75, p = < 0.0001). However, we are concerned with the accuracy of measuring the 2D:4D ratio. In these data F = 64.75, p = < 0.0001 for 2D:4D. That is, differences in 2D:4D ratio between subjects are 65 times greater than its measurement error. A further indication of the accuracy of the measurement of the 2D:4D ratio is given by the intraclass correlation coefficient or repeatability (r1) of the measurements. This is calculated as follows: r1 = Groups MS - Error MS/(Groups MS + Error MS) For the 2nd digit r1 = 0.99, for the 4th r1 = 0.99, and for the 2D:4D ratio r1 = 0.97. Therefore 2D:4D may be measured accurately.
Of course, measurement of digit length can be performed in a number of ways. For example, X-rays of hands may be used and measurements made from the proximal tip of the proximal phalanx (i.e., the finger bone which is closest to the palm) to the distal tip of the distal phalanx. Figure 1.2 shows the relationship between 2D:4D ratios calculated from measurements from X-rays and from soft tissue from 136 Jamaican children. The 2D:4D ratios correlate quite well. This is despite the fact that the soft-tissue measurements of the digits were taken from approximately halfway along the proximal phalanx whereas bone measurements began at the proximal end of the phalanx. In addition, the X-rays were taken 2.5 years before the photocopies. That there were significant relationships between the 2D:4D ratios from the two types of measurements also suggests that the ratio remains stable in children across periods of rapid growth.
It is not usually practical to measure 2D:4D ratio from X-rays. In general in this book I refer to soft-tissue measurements taken directly from the hand using vernier calipers or made from photocopies of the palm of the hand or from "ink prints" of the ventral surface of the phalanges. Comparisons between 2D:4D ratios from the hand and from photocopies of the hand have shown the latter method to produce similar results. For example, the 2nd and 4th digits of 30 hands were measured from the hands and from photocopies of the hands. The 2D:4D ratios calculated from these two methods showed significant similarities (r1 = 0.88, F = 15.86, p = 0.0001).
Sex Differences and Their Ontogeny
Many traits showing sex differences become obvious at puberty, e.g., height and weight differences; dimorphisms in jaw size and thickness of brow ridges; and hip, waist, and shoulder differences. Before puberty there may be little in the way of sex differences.
Puberty and Sex Differences
While breasts are thought to be typical of women, prenatal breast development is identical in males and females. The breast forms from the mid- thoracic persistence of the ectodermal mammary ridge, which initially extends on each side from the base of the forelimb to the hind limb (Sadler 1985). An area in the mid-thoracic region penetrates the underlying mesenchyme to give rise to the lactiferous ducts. The ducts empty into an epithelial pit from which the nipple forms. These changes are the same in males and females. In fact at birth 80% of both male and female babies produce milk. No further development occurs until puberty when estrogenic changes stimulate marked breast development in females. Approximately 38% of males may show early pubertal gynecomastia or breast enlargement as testicular estrogen increases before adult concentrations of testosterone reach maximum levels. However, pubertal gynecomastia resolves in 92% of boys by 16 years. The sexes are then markedly dimorphic in their breast development until the onset of senile gynecomastia in middle age when around 30% of men show breast enlargement, rising to 60% by the seventh decade (Gateley 1998).
Similar temporal patterns of sexual dimorphism may be seen in the skeleton and are detailed by Knight (1991). In the skull there are a number of features that become accentuated at puberty but are modified with age so that discriminant function analysis may best determine sex between approximately the twentieth to the fifty-fifth year. The male skull is more rugged and has greater development of muscle ridges than the female skull. Compared to females the male supraorbital ridge, mastoid process, palate, teeth, and mandible are larger. The male mandible has a squarer symphysis or chin region; the angle between the body and the ramus is more upright; the condyles and the ramus and the coronoid process are larger than those in females.
Prepubertal Sex Differences
Unlike the skull, sexual dimorphisms in the pelvis may be seen in weak form in children and even at the fetal stage. Males have a more rugged pelvis for muscle attachments with higher iliac blades and a narrower suprapubic angle. Many of the sex differences in the pelvis relate to childbirth. For example, the pelvic inlet is more circular in females, while that of the male is more heart-shaped as the result of the position of the sacrum. The sacrum is wider in females with a shallower curve, while the curve in the male is continuous and the coccyx may even project forward. After puberty the relatively wider pelvis in women leads to a difference in the angle that the shaft of the femur makes with the vertical. That is, the slope of the shafts is greater in women in order that the lower ends of the femur sit horizontally on the tibia.
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Table of Contents

Table of contents for DIGIT RATIO: A Pointer to Fertility, Behavior, and Health by John T. Manning

List of Figures
List of Tables
Preface and Acknowledgments
Sex and Population Differences
Associations with Testosterone and Estrogen
Assertiveness, Status, Aggression, Attractiveness, and the Wearing of Rings
Reproductive Success and Sexually Antagonistic Genes
Hand Preference, Verbal Fluency, Autism, and Depression
Birth Weight, Heart Attack, Breast Cancer, and Sex-Dependent Diseases
Male and Female Homosexuality
Music, Musicians, and Mate Choice
Sporting Ability, Running Speed, Spatial Perception, Football Players, and Male Competition
2nd to 4th Digit Ratio and Future Research
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