[update: see also my post on race and sexual behaviour]
Rushton, in Race, Evolution, and Behaviour (1995), makes various claims about the differing rates of maturation between races. I want to get an idea of the current state of these claims. The areas in which Rushton claims different rates can be grouped into the following:
– Gestation period
– Psychophysical development
– Ossification (incl. dental development)
– Sexual Debut
The black/white difference in gestation period has been confirmed in various studies in the U.S. (sources: 1, 2) Britain (sources: 1, 2), and African countries (sources: 1, 2). Recent studies have found it plausible that at least part of the cause is genetic (source), and researchers have begun looking for the specific genes involved (source). Although environmental explanations are still being sought (source). Additionally, there is evidence that, despite black infants being born ‘prematurely’, they have experienced accelerated maturity:
“The suggestion that maturity occurs earlier in gestation in Black babies is consistent with many reports over the last 20 years that black babies suffer less from respiratory distress syndrome than white European babies for any given gestation at birth… Consistent with the accelerated pulmonary maturity seen in preterm black babies, neonatal survival in black babies is also higher than that of white Europeans.”
Rushton cited studies showing faster black infant development on various indexes, including sitting up, walking, turning over, etc. We should also discuss Richard Lynn’s formulation of ‘black infant precocity’ (source) – which essentially posits that black infants are more advanced psychophysically until around 15 months, after which point the lead disappears. A number of studies over the last decades have found a pattern of black infant precocity in psychophysical development. A 2007 study from South Africa entertains Lynn’s idea:
“Another possible explanation for the findings of the current study is the concept of Black infant precocity (Lynn 1997). Lynn compared Black South African infants’ performance on the Bayley Scales of Infant Development to the American standardisation sample and found that the Black infants were significantly more advanced in terms of their mental and motor development during approximately the first 15 months of life. This age range is similar to that of the sample in the present study (i.e. 13 to 16 months). Positive evidence of this was first advanced by Falade (1955) who found that Black Senegalese infants assessed on the Gesell Developmental Screening Inventory were significantly more advanced in areas of fine-motor development, eye-hand coordination, problem-solving and object permanence than matched White American infants. Similar results were obtained with Ugandan infants (Gerber 1958), Nigerian infants (Freedman 1974) and Black South African infants (Richter-Strydom and Griesel 1984, Lynn 1997).”
Nevertheless, genetic explanations have not swept the field; from a 2011 paper:
“This “precocity” was initially interpreted as a biological, genetically driven phenomenon. Subsequent investigation led in other directions (Kilbride & Kilbride, 1975; Leiderman, Babu, Kagia, Kraemer, & Leiderman, 1973; Super, 1976, 1981; Varkevisser, 1973). First, it was demonstrated that traditional methods of infant care common in sub-Saharan Africa include deliberate teaching and practice of sitting and walking (and, sometimes, crawling). These customary practices, carried out by parents, siblings, and other relatives, reflect a local understanding of what young children are capable of, and this understanding is manifest from the infant’s earliest days (Super & Harkness, 2009). Further, careful observation revealed high levels of leg, trunk, and back exercise, and also vestibular stimulation, incidental to customary methods of holding and carrying the infant… When families migrate from traditional rural areas to an urban environment such as Nairobi, they adapt to quite different physical and social settings, and they come in contact with a greater variety of ethnotheories: Both daily life and infant motor development shift toward the Euro-American pattern. (source)
(Chapter 4 of Handbook of cultural developmental science also offers an in-depth cultural explanation to differing rates of motor development). Certainly culture has an important role to play in the variation of psychophysical maturation, but one can also point to the fact that African Americans likewise consistently show motor precocity (sources: 1, 2, 3, 4, 5), as do Caribbeans (source). And one might also wonder why Africans show such consistent ‘local’ understandings (see references to Africa wide findings above).
Some studies have shown differences between whites and asians as well. There is a good discussion of past results in Motor Development in Canadian Infants of Asian and European Ethnic Origins (2009), which itself found no difference between asians and whites in Canada. Studies of asians have either shown no difference with whites, or slower development than whites, whereas studies comparing blacks have all (as far as I’m aware) found faster psychopysical development.
It is suggestive that the results have been broadly consistent with Rushton’s pattern of blacks > whites > asians. However, the varying results of asians vs. whites suggests environmental factors (at least for those races). The asian case especially implicates culture, because it tends to be in the studies in which asians and caucasians live in the same country that they become more, or entirely, similar (source).
Connolly et al. (2011) in “The Influence of Ethnicity on Infant Gross Motor Development: A Critical Review” analyze past studies and find that “In the articles reviewed, the differences in motor milestone attainment between Black infants and Western norms averaged 1 month… As well, in ethnic groups that were delayed, the amount varied from 1-1.5 months compared to Western norms… Currently, we can only say that motoric differences exist between ethnicities but we cannot… explain accurately why and how these differences exist.”
Ossification rates (including dental development)
Earlier studies showed accelerated rates of ossification in early childhood for Africans, followed by retardation at later ages, which fits in with the ‘Black Infant Precocity’ theory. From Effects of ethnicity on skeletal maturation: consequences for forensic age estimations (2000):
“Hand skeleton development of a population of West African children aged between 10 days and 15 years was investigated by Massé and Hunt . In comparison to children studied by Greulich and Pyle, they found early maturity in the early postnatal months, followed by deceleration and sometimes retardation in middle and advanced childhood. Marshall et al.  and Garn et al.  also reported comparatively accelerated skeletal development in Africans during their early years of age. On the other hand, there have been several studies which reported that in advanced childhood and adolescence no time difference in skeletal maturation existed between Whites and Blacks… Studies so far evaluated seem to suggest that there is a genetically determined potential of skeletal maturation which does not depend on ethnicity and is available for exploitation under optimum environmental conditions (i.e. high socio-economic status), whereas a less favourable environment may lead to retardation of skeletal maturation.”
This, however, is not really contradictory to the black precociousness position, which generally argues a faster development in earlier childhood, and prenatally, not a continuously accelerated pace, while the counter-evidence comes from later childhood. Rushton has, however, also generally ascribed precocity for blacks to a longer time period, not just in infancy (ie. through puberty). The early infancy data, sparse as it is, indicates possible black infant precociousness. Unfortunately, I don’t know of any newer studies on early racial differences in ossification rates. However, the above cited evidence on black infant precocity in fetal development and motor development, provides at least a background which is generally supportive of what limited data is available.
The third molar shows evidence of signficant differences in age of mineralization among races (source), black development stages being earlier than other groups, in the order black > white > asian.
The U.S. data generally shows earlier age of puberty for blacks (source), but given how sensitive puberty is to environmental factors, and the lack of a global pattern, genetic explanations seem premature. Menarchy, for example, varies greatly among blacks depending on country, as well as other environmental factors such as urban vs. rural, SES, etc. Age of menarchy differs dramatically in general over time, and, in fact, between European countries as well..
Nevertheless, persistent racial differences in the U.S. lead some to consider genetic explanations:
“…controlling for height and either BMI or weight, the rate of early menarche remained significantly higher among black girls, suggesting that race is an independent factor of pubertal/menarcheal timing. The racial difference in pubertal maturation may reflect genetic factors. Black girls present higher insulin response to a glucose challenge, and subsequently increased levels of free IGF1, which is associated with skeletal and sexual maturation compared to white girls .” (source)
Once again, the U.S. data shows earlier black sexual debut (and later for asian) – (sources: 1, 2). However, blacks in America seem to begin sexual activity at a younger age than blacks in Africa (sources: 1, 2, 3). Thus, data from the U.S. alone is likely misleading For example, in data from the U.S. we see “the median reported age of first sexual intercourse was 15.0 years for Black males, 16.3 years for Black females, 16.6 years for white males and females, 16.5 years for Hispanic males, 17.3 years for Hispanic females, and 18.1 years for Asian American males.” However, Sexual behaviour in context: a global perspective (2006) has global data on median age of first sex for men and women – and when I average out the median age of first sex for Sub-Saharan countries, I get 18.5 years for men, and 16.7 years for women (average 17.6 years – which is pretty close to what we get from the average of 7 western nations: 17.75).
Britain provides a good example of the variability because there are large numbers of both black Africans and black Caribbeans. A 2005 study found that, while white men began sexual activity at a median age of 17, for black Caribbeans the age was 15; however, black Africans began, like whites, at 17. (Indians and Pakistanis at 20). For women, whites and black Carribeans began at 17, while black Africans at 18 (Pakistanis: 20, Indians: 21). Similarly, a much larger percentage of black Caribbean males claimed to begun to have sex before age 16, whereas the number were about the same for white and black African males. For women, about the same percentage of white and black Caribbean women began having sex before 16, whereas significantly less black African women did. It should be noted that black African immigrants are often an elite sampling of their home country. It is of interest as well that Caribbeans have such an early sexual debut, similarly to American blacks – interesting because the global data referenced before does not show the Caribbean to have lower age of sexual debut. So what’s going on?
This last topic especially I’m going have to revisit in the future. Clearly, very little is settled in any of them.