Library of Excerpts

Monozygotic and Dizygotic Twins, Heterochrony and Maturation Rates

“Plasma gonadotropin levels throughout the regular menstrual cycle in 10 Japanese women were measured daily using radioimmunoassay. At the peak of ovulation, mean FSH levels were 17.6 +/- 7.9 mlU/ml and mean LH levels were 75.2 +/- 26.0 mlU/ml. At midcycle, the mean gonadotropin levels were significantly lower in Japanese women than in Nigerian women who, as reported by Nylander (1973), had a high frequency of twinning. It is, therefore, suggested that the low frequency of dizygotic twinning in Japanese women might be related to their low output of gonadotropin. “ (Soma H, Takayama M, Kiyokawa T, Akaeda T, Tokoro K (1975) Serum gonadotropin levels in Japanese women. Obstet Gynecol 46 (3): 311)

"To continue along same line in an example involving human beings, certainly no one would have guessed that the two men shown in Figure 12-5 are identical twin brothers. As described by James Shields (1962, pp. 43-44, 178-180) and in Tanner (1978, p. 119), they were separated at three weeks after birth and they underwent very different rearing regimens. They exhibit well the enormous developmental potential for variation that usually remains unexpressed in closely related people." (Gilbert, G (1992) Individual Development & Evolution. Oxford Univ. Press: New York p. 153)

"In this total population of 21 pairs of same-sexed twin pairs, the concordance rate for autism was 36% in the monozygotic pairs and 0% in the dizygotic pairs." (Folstein SE, Rutter ML (1988) Austism: Familial aggregation and genetic implications. J Autism and Developmental Disorders 18: pp. 10)

"In their review of family studies of early onset psychoses, Gottesman and Shields (1982) calculated that 2.2% of siblings of autistic children also have the condition. August, Stewart and Tsai (1981) found 2 out of 71 siblings of autistic children to be themselves autistic (language delay, gross expressive or receptive language abnormality, a large verbal/performance discrepancy on IQ tests, specific learning difficulties and the need for special education) were found in 15.5% of siblings of autistics but in only 3% of siblings in the control group. But the best evidence so far for a genetic factor as a contributing cause of autism at least in a proportion of cases, comes from Folstein and Rutter (1977). Of 11 MZ twin partners of autistic boys, 4 were autistic and another 5 had cognitive and social deficits short of autism; while among 10 DZ twin partners none were austistic and only one had other deficits. Moreover, among 17 discordant twin pairs, there were 12 pairs in which the autistic twin had been exposed to a birth hazard while the non-autistic twin had not. So it may be that for autism to develop, brain damage has to occur in the setting of a genetic predisposition." (Wolff S, Narayan S, Moyes B (1988) Personality characteristics of parents of autistic children: A controlled study. J Child Psychology and Psychiatry 29: 144)

"The twin with a lower non-verbal I.Q. but higher verbal I.Q. made much more progress in both social relationships and use of languge. In the fourth pair (4) there was a 39 point I.Q. difference; in this case, the more intelligent twin was less severly autistic, although the type of behavior was closely similar in both. It is also notable that the more intelligent twin did not develop autism until 3 years of age, although apart from the late onset the clinical picture was typical of autism. ... The results are striking. Nine of the 11 MZ pairs were concordant for some kind of cognitive disability, usually involving language, whereas this was so for only one out of the 10 DZ pairs..." (Folstein S, Rutter M (1977) Infantile autism: A genetic study of 21 twin pairs. J Child Psychology and Psychiatry 18: 303)

"We identified five features know to be associated with brain damage (and hence likely to to predispose to autism): severe haemolytic disease (Gerver and Day 1950), a delay in breating of at least 5 minute after birth (Drage and Berendes, 1966); Hunter, 1968), neonatal convulsions (Rose and Lombroso, 1970), a second birth which was delayed be at least 30 minutes following the birth of the first twin (Dunn, 1965; Kurtz et al., 1955) and multiple congenital anomalies. Such features were present in 11 out of 42 children. ... In order to examine these 11 discordant cases further, a wider definition of biological hazard, in terms of a marked diffeence between the twins, was employed. This included a birth weight at least a pound less that the other twin (three cases) (Willerman and Churchill, 1967), a patholigically narrow umbilical cord (one case), a more severe haemolytic anaemia associated with neonatal apnoea (two cases), and a severe febrile illness possibly involving encephalitis (one case). This differentiated a further six cases (see Table 7), and again it identified the austic one each time. It may be concluded that some form of biological impairment, usually in the perinatal period, strongly predisposed to the development of autism." (Folstein S, Rutter M (1977) Infantile autism: A genetic study of 21 twin pairs. J Child Psychology and Psychiatry 18: 304-5)

"A systematic study was made of a representative group of 21 same-sexed twin pairs (11 MZ and 10 DZ) in which least one twin shoed the syndrome of infantile autism. There was a 36 per cent pair-wise concordant rate for autism in MZ pairs compared with 0 per cent concordance in DZ pairs. The concordance for cognitive abnormalities was 82 per cent in MZ pairs and 10 per cent in DZ pairs. It was concluded that there were important hereditary influences concerning a cognitive deficit which included but was not restricted to autism. In 12 out of 17 pairs discordant for autism, the presence of austism was associated with a biological hazard liable to cause brain damage. It was concluded that brain injury in the infancy period may lead to autism on its own or in combination with a genetic predispostion." (Folstein S, Rutter M (1977) Infantile autism: A genetic study of 21 twin pairs. J Child Psychology and Psychiatry 18: 310)

"Reduced shift implies that twins are slightly less biased toward dextrality than nontwins such that, when the incidence of left-handedness in the general population is 7 or 8%, about 11 or 12% is expected in twins." (Annett, M. (1996) In defence of the right shift theory. Perceptual Motor Skills 82 (1): pp. 119)

"There are sex differences in R-L hand skill such that females are slightly more right shifted than males. Females tend to be more mature at birth than males. This observation, together with the finding of a lesser shift in twins than in nontwins, suggested the important hypothesis that cerebral asymmetry depends on rate of hemispheric maturation in fetal life." (Annett, M. (1996) In defence of the right shift theory. Perceptual Motor Skills 82 (1): pp. 120)

"If development was slower (in twins or in males), differences between the hemispheres might be reduced. Chi, Dooling, and Gilles (1977) found, in a study of fetal brain growth, that twins were delayed by two of three weeks in the appearance of cerebral convolutions in comparison with nontwins. No difference were observed in this study between the sexes. " (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 304)

[citations removed] "With regard to twins, the expression of the rs + gene is less effective in twins than the singleborn, probably because the rate of growth is slowed in utero, and this is true for both monozygotic (MZ) and dizygotic (DZ) pairs. The prediction that twins are less biased to dextality has been strongly supported for hand skill and for hand reference. The widely-held assumption that a genetic influence on handedness requires MZ and DZ twins to differ for the distribution of RR, RL and LL pairs is mistaken. According to RS theory, almost all of the variability for handedness in both types of twin is due to chance, while the genetic influence is virtually constant." (Annette, M. & Alexander, M.P. (1996) Atypical cerebral dominance: predictions and tests of the right shift theory. Neuropsychologia 34 (12): 1225)

"It may be concluded that twinning and sex are associated with differences in neonatal maturity, in rate of language acquisition, and in incidences of right-handedness. All of these differences are consistently in the direction expected if the expression of the rs+ gene is correlated with neonatal maturity; that is, greater in females than males and greater in the singleborn than in twins. The postulate that twins have a reduced right shift now seems an essential part of the RS theory and not at all "ad hoc". (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 305)

"The differences in dizygotic twin frequency, and presumably ovulaton rate, are in the same direction as the differences in testis size. The frequencies of dizygotic twins are even higher (up to 49 per 1,000 births) among African blacks. ... Yoruba women, with the world's highest frequency of dizygotic twins, have higher FSH and LH levels at the time of ovulation than do Japanese women, who have the lowest frequency of dizygotic twins. This variation in female hormone levels may contribute to the distribution of the incidence of breast cancer, which is known to be related to oestrogen levels. Even after all other risk factors for breast cancer have been taken into account, the incidence among Japanese women remains inexplicably low. Perhaps this puzzel, the so-called 'Japanese factor' of (breast cancer, is related to the low double-ovulation frequencies and low hormone levels." Diamond, JM (1986) Variation in human testis size. Nature (London) 320: 488-489)

"further evidence for a genetic aetiology came following Susan Folstein and Michael Rutter's systematic epidemiological study of same sex British twins, which showed a pairwise concordance rate for autism of 36% amongst monozygotic (MZ) twins compared with 0% in dizygotic (DZ) twins (Folstein & Rutter, 1977, a,b). In addition, the results from this study suggested that the genetic liability for autism also predisposed individuals to a lesser variant of the condition partly indexed by a variety of language-related cognitive disorders (such as marked language delay, severe articulation disorder, and pronounced reading retardation). Redefinition of the phenotype to include these indices of the lesser variant raised the pairwise concordance rate in MZ and DZ twins to 82% and 10%, respectively." (Bolton P, MacDonald H, Pickles A, Rios P, Goode S, Crowson M, Bailey A, Rutter M (1994) A case-control family history study of autism. J Child Psychology and Psychiatry 35: 878)

"While the monozygotic twinning rate is nearly constant at about 3 1/2 per 1000 in all groups, dizygotic twinning (the r-strategy) is greater among lower than upper social class women in both European and African samples (Golding, 1986; Nylander, 1981). Also, the rate per 1000 births among Mongoloids is <4; among Caucasoids, 8; and among Negroids, >16, with some African populations having rates as high as 57 per 1000 (Bulmer, 1970). The incidence of nonmonozygotic triplets and quadruplets shows comparable rank orders. Moreover, data from racially mixed matings suggest that the mother independently of the race of the father, as shown for Mongoloid-Caucasoid crossings in Hawaii, and Caucasoid-Negroid crosses in Brazil (Bulmer, 1970)." (Rushton, J.P. & Bogaert, A.F. (1987) Race differences in sexual behavior: Testing an evolutionary hypothesis. Journal Research in Personality 21(4): pp. 534)

"Indeed, there is evidence that at 19 to 32 weeks gestation the development of various sulci and gyri is delayed by two to three weeks in twins compared to singletons (Gilles et al. 1983). Annett noted that language development is often delayed in twins and she argued that this too was an indication of delayed maturation. It would be interesting to see whether a raised frequency of left-handedness is particularly striking among twins who are late to talk... ...Boklage (1981) argued that non-right-handed parents are more likely than right-handed parents to produce twins. He asked parents of twins to specify the handedness of themselves, their twins and other family members. A singleton control group was not used, but the rate of left-handedness reported in twins (around 20 per cent for both MZ and DZ) was higher than in studies assessing singletons by comparable means. Furthermore, the rate of left-handedness in first-degree relatives was not significantly different from that in twins (18 per cent in brothers, sisters and fathers, and 13 per cent in mothers). Much lower rates of left-handedness were reported in second-degree relatives of twins." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 38)

"Teng et al. (1976) pointed out that cases of pathological left-handedness will constitute a higher proportion of the population of all left-handers in countries such as China where there is strong social pressure to be right-handed. They found that twinning was associated with both decreased right-handedness and reduced college entrance is a large Taiwanese population, which is consistent withthe view that there is an excess of left-handedness among twins due to the influence of pathological factors. This suggests that if we could find some independent way of discriminating between pathological and natural left-handers in Wesern cultures (see p.95), we might be able to obtain clearer evidence for different distributions of handedness in twins and singletons. However, against this view is Annett's (1985) finding of a significant excess of left-handers among twins as compared to singletons even when probable pathological left-handers (those scoring more than two standard deviations below the mean on a test of manual dexterity) were excluded from consideration." (Bishop, D.V.M. (1990) Handedness and Developmental Disorder. MacKeith, Manchester pp. 36)

"In twins and their first-degree relatives the rate of lefthandedness is higher than in second-degree relatives (Boklage 1981). This result is consistent with older finding of higher rates of lefthandedness in twins and their singleton relatives than in the general population." (Geschwind & Galaburda 1987: 3, Cerebral Lateralization)

"Twinning in Nigeria increases with both age and parity (Bulmer 1970; Nylander 1978). Some congenital anomalies, such as neural tube defects, also vary with age and parity." (Geschwind & Galaburda 1987: 137, Cerebral Lateralization)

"Even early studies of the genetics of handedness revealed an excess of lefthanders among twins and, even more surprising, among their relatives. The recent work of Boklage (1984) has shown that the rate of lefthandedness in twins (both monozygotic and dizygotic), and in their mothers, fathers, and siblings, is double the rate in the siblings of the parents. This would appear to support the belief that twinning and lefthandedness are associated, since otherwise one would have to argue that the second-degree relatives of twins had lower rates than the general population. the elevated rate of lefthandedness is both typess of twins and their first-degree relatives has no simple genetic explanation and may reflect nongenetic influences." (Geschwind & Galaburda 1987: 138, Cerebral Lateralization)

"The rate of monozygotic twinning is very much the same throughout the world: about 3 twin births per 1000. The dizygotic twinning rate, however, varies considerably. Thus, some older data showed that in Japan there was approximately 1 dizygotic twin birth out of 165, whereas in the northern countries of Ireland and Scotland the rate was about three times as high. The rates in northern Europe are generally of this order, and those in southern Europe are lower (Bulmer 1970). The highest twinning rates so far observed occur in West Africa, particularly among the Yoruba, especially in the vicinity of Ibaden, where the rate is about 1 in 18 births. In young Yoruba women of high parity the rate of twinning may be nearly 1 in 10 births. It is no surprise that twin dolls are a characteristic feature of Yoruba art. In the United States, Blacks (most of West African descent) have higher twinning rates, and Orientals have lower twinning rates than Caucasians (Bulmer 1970).

"There is also a high rate of neural tube defects in many populations in which dizygotic twinning is common, as it is in Ireland and Scotland. In China, where the rate of dizygotic twinning is very low, neural tube defects occur about one-tenth as frequently as in Ireland (Ghosh et al. 1981). It is very possible that the rate of twin conceptions is higher than is generally appreciated, since in some cases one fetus may die and be reaborbed. The rate of twinning has been falling in recent years in Europe, as has the rate of neural tube defects, data for which no satisfactory explanation has yet been found." (Geschwind & Galaburda 1987: 140-41, Cerebral Lateralization)

"Another result in keeping with the hypothesis is the reported higher rate of dyslexia among twins. (Bakwin 1973). (Geschwind & Galaburda 1987: 142, Cerebral Lateralization)

"Neural tube defects are in general more common in populations with high rates of twinning, a condition associated with an elevated rate of lefthandedness. In fact, there is a high rate of twinning among the parents of children with spina bifida (LeMarec et al. 1978). Fraser (1983) has also found a high rate of lefthandedness in families of children with spina bifida." (Geschwind & Galaburda 1987: 166-7, Cerebral Lateralization)

"What they found was that among fraternal twins where one of the twins had been diagnosed as being hyperactive, the incidence of both twins being hyperactive as 33%. Among the identical twins, however, it jumped up to 51%. So clearly, even something as apparently baseline as hyperactivity contains both a nature and a nurture aspect." (Hartmann 1996: 81, Beyond ADD)

"Whether for reasons of pathology (fetal crowding, competition, etc.) or of mirror imaging, sinistrality is said to be more common in twins than in singletons (for reviews, see Brolage, 1980; Springer & Searleman, 1980). The general consensus is that the incidence of nondextrality is the same in both monozygotic and dizygotic twins and is twice that of singletons." (Bradshaw & Nettleton 1983: 203, Human Cerebral Asymmetry)

"There was no change in the MZ twinning rate during World War II; however, the DZ rate decreased in France, Holland, and Norway during this period where undernutrition was common (Bulner 1970). (Bernds, W.P. & Barash, D.P. (1979) Early Termination of Parental Investment in Mammals, Including Humans In Evolutionary Biology and Human Social Behavior. N. Chagnon & W. Irons, eds. Pp. 504)

"It has long been established that light stimulation has an inflence via the pituitary on the reproductive system (Le Gros Clark et al., 1939). So the seasonal variation in DZ twinning rates (James, 1980c) is probably at least partially caused by seasonal variation of light stimulation and hence of gonadotrophin levels. So it is satisfactory that the seasonal variation of DZ twinning is synchronous with the seasonal variation in sex ratio (James, 1984b). (James, W. H. (1986) Hormonal control of the sex ratio. Journal of Theoretical Biology 118: 427-441.)

"If twinning can be seen as a comparable effect, it is worth pointing out that sheep have long been known to be much more likely to give birth to twins if ewes are fed a high-calorie diet for a week or two prior to mating." (Badcock, C. (1991) Evolution and Individual Behavior: An Introduction to Human Sociobiology Oxford: Blackwell. pp. 161)

"If development was slower (in twins or in males), differences between the hemispheres might be reduced. Chi, Dooling, and Gilles (1977) found, in a study of fetal brain growth, that twins were delayed by two of three weeks in the appearance of cerebral convolutions in comparison with nontwins. No difference were observed in this study between the sexes. " (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 304)

"It may be concluded that twinning and sex are associated with differences in neonatal maturity, in rate of language acquisition, and in incidences of right-handedness. All of these differences are consistently in the direction expected if the expression of the rs+ gene is correlated with neonatal maturity; that is, greater in females than males and greater in the singleborn than in twins. The postulate that twins have a reduced right shift now seems an essential part of the RS theory and not at all "ad hoc". (Annett, Marian (1985) Left, Right, Hand and Brain: The Right Shift Theory London: Lawrence Erlbaum pp. 305)

"Nevertheless, their review of the twin studies on handedness gave figures which allow some doubt: the rate of handedness-discordance in monozygotic twins (2900 pairs) is 21.7% and the rate of handedness discordance in dizygotic twins (2589 pairs) is 22.6% (p. 121). The suggestion that there is almost no empirical support for the specific "testosterone hypothesis" is convincing; it should not lead to rejection of all prenatal non-genetic models of cerebral lateralization." (Dellatolas, G. (1994) Birth order and month of birth are not related with handedness in a sample of 9370 young men. Cortex 27 (1): 196)

"The ethnographic record {e.g., A French Army Surgeon (1898/1972), a 30-year specialist in genitourinary diseases} makes reference to numerous anatomical distinctions which show a similar pattern of whites being between blacks and Orientals. These include the placement of female genitals (Orientals front and high; blacks back and low); angle and texture of erection (Orientals parallel to body and stiff, blacks at right angles to body and flexible); salient buttocks, breasts, and muscularity (Orientals least, blacks most); and size of genitalia (Orientals smallest, blacks largest). We averaged the ethnographic data on erect penis and found the means to approximate: Orientals, 4 to 5.5 in. in length and 1.25 in. in diameter; Caucasions, 5.5 to 6 in. in length and 1.5 in. in diameter; blacks, 6.25 to 8 in. in length and 2 in. in diameter. Women were proportionate to men, with Orientals having smaller vaginas and blacks larger ones, relative to Caucasians. Clitoral size differed in length: in European women, 1.2 in.; in African women, 2 in. variations were noted; in French West Indies, the size of the penis and vagina covaried with amount of black admixture; Arab men, who were often mixed with black, had larger penises than Europeans. Recent data show similar patterns. Measurements taken from living subjects as well as those at autopsy, show the size of testes is twofold lower in Asian men than Europeans (9 g vs 21 g), a difference too large to be accounted for entirely in terms of body size (Diamond, 1986; Short, 1984). Concomitantly, as mentioned, Asian women have lower ovulation rates than Caucasian women, as indexed by dizotic twin frequency, with the frequency per 1000 across several Asian populations being < 4, while for Caucasians it is 8, and for blacks 16 per 1000 (Bulmer, 1970; Diamond, 1986). Contrary to the general trend, Freeman (1934) observed that, at autopsy, American blacks had less heavy testes than American whites (13g vs 15g). Freeman (1934), however, did find that black women had heavier ovaries than white women. Subsequently Daniel, Fienstein, Howard-Peebles, and Baxley (1982) found no black-white difference in testicular volume among American adolescents, while Ajmani, Jain, and Saxena (1985) found larger scrotal circumference in Nigerians than Europeans (212.6 mm vs 195.1 mm or 8.37 in. vs 7.68 in.). A French Army Surgeon (1988/1972) also provided early observations that, in speed of sexual maturation, Orientals < whites< blacks. Several subsequent studies are confirmatory. In the United States, blacks are more precoscious than whites as indexed by age at menarche, first sexual experience, and first pregnancy (Malina, 1979). A national probability sample of American youth found that by age 12, 19% of black girls had reached the highest stages of breast and pubic hair development, compared to 5% of white girls (Harlan, Harlan, & Grillo, 1980), although the same survey found white and black boys to be similar (Harlan, Grillo, Coroni-Huntley, & Leaverton, 1979). Subsequently, Westney, Jenkins, Butts, and Williams (1984) found that 60% of 11-year-old black boys had reached the stage of acelerated penis growth in contrast to the white norm of 50% of 12 1/2-year-olds. This genital stage significantly predicted onset of sexual interest, with 2.2% of the black boys experiencing intercourse by age 11. While some surveys found that Oriental girls enter puberty as early as whites (Eveleth & Tanner, 1976), others suggest that in both physical development and onset of interest in sex, the Japanese, on the average, lag 1.5 to 2 years behind white Americans (Asayama, 1975). (Rushton, J.P. & Bogaert, A.F. (1987) Race differences in sexual behavior: Testing an evolutionary hypothesis. Journal Research in Personality 21(4): pp. 536-7)