HVGIQ: Jamaica

IQ and the Wealth of Nations (2002) cites one study for Jamaica (Manley, 1963 ). The sample size is nearly two thousand school children, and the reported IQ is 72 (p. 209). Lynn’s various updates to that book have included five additional references. However, in another anomaly, the most recent book contains only two references, both with relatively small sample sizes, and omits the large study altogether. The IQ estimate for Jamaica has also been lowered to 67! (Lynn, 2012, p. 403)

Here I review over 20 intelligence test studies from Jamaica. I find that Lynn’s numbers were not accurately reported, and that IQ is significantly higher in Jamaica than his books have claimed.
 
 
Section I: Summary of 24 references
Section II: Tabulated IQ data from 20 references
Section III: Lynn’s data and an updated estimate of Jamaican IQ
 
 
This post is divided into three sections. The first (and longest) section provides summary information for 24 references. If these dry details are of limited interest, you can skip ahead to the second section, which simply lists the numbers, or to the third section, which criticizes Lynn’s treatment of Jamaican IQ data, and provides an updated average.

 
 
                    SECTION I: SUMMARY OF 24 REFERENCES                    
 
 
Race Crossing in Jamaica (Davenport & Steggerda, 1929 )

Charles B. Davenport was one of the most prominent American biologists of the early 20th century. According to science historian William Provine (1973): “… Davenport was the first geneticist to devote considerable attention to problems of human heredity … American geneticists recognized him as the leading student of human heredity… (p 791)”. Davenport was one of the leading figures in the eugenics movement, and had scientific and political concerns about the hereditary consequences of race mixing. To collect better data on this issue, Davenport organized a large study in Jamaica to make physical, social and psychological measurements of the island’s mulatto population. The results were published in his most (in)famous book, Race Crossing in Jamaica (Davenport & Steggerda, 1929), which probably contains the earliest intelligence test data for this Caribbean nation.

In 1926 Davenport collected data for 300 adults, ages 18-40, and some 1465 children, ages 10-16. 100 adults were chosen from three racial groups: white, black, and mixed (“brown”). The subjects were all intentionally selected from the lower, agricultural social class, and thus somewhat matched for status. Consequently, most of the island’s whites, who worked in government and commerce, were excluded from the study, and Davenport had to travel to the nearby Cayman Islands to complete his white sample. Cayman Islanders therefore comprise about half of the white sample.

The researchers decided the race of the participants by visible ancestry, physical measurements, reported genealogy, interviews, and local informants. (Census figures from 1921 indicate the population was 76.9% black, 18.3% mulatto, 1.7% white, and 3% Asian/other (p. 7)).

Twelve different types of ability tests were given to the sample. Many of these tests were not administered according to procedure, and do not permit comparison with the standardization populations, but both the Goodenough Draw-a-Man test and the Army Alpha Test appear to have been administered and scored in a manner that permits comparison with US whites.

I converted the raw scores on the Draw-a-Man into deviation IQs using the norms by Harris (1963). The Alpha scores of the adult sample were converted to IQ using the chart by Bregman (1926). For ages 10-16, I used the US schoolchildren tested by Doll (1919) as the comparison group. The results are shown in Table 1:

Scores on the Draw-a-Man test are extremely low, while scores on the Alpha Test are not too far behind the US norms. This is surprising because the Alpha Test is more culturally loaded. Nevertheless, both tests reveal a similar racial gap: blacks score about 1/4th of a standard deviation below whites (d = .25), and the browns are about midway between the blacks and the whites. This is lower than the .40 B-W gap found in the Cayman Islands, however, the Jamaican sample is less representative of the island’s white population.

A weighted average of the two tests gives us an IQ of 68 for Jamaica in 1926.

Grinder et al., 1964

Black, white, and mixed race school children were given the Draw-a-Man test once again around 1940. Margaret Wooster Curti administered the test to 941 children, ages 8-11. The researchers were wary of the results and only published a very cagey summary of what they found, nearly 25 years after they had collected the data.

The data reveals a large racial IQ gap, but the authors do not report the scores, which are said to be very low compared to the US norm group. The authors likewise do not analyze the data to determine the magnitude of the racial gaps; they perform a chi-square analysis, which tests the statistical robustness of the gap: “The results… suggest that there is a strikingly strong association between skin color and intelligence as measured by the Goodenough Draw-a-Man Test (p. 185)”.

According to Richard Lynn in the Global Bell Curve (2008): “It is possible from the data they presented to calculate IQs for their three groups in relation to an IQ set at 100 for the total sample. The resulting IQs are 104.5 for the light group, 101.0 for the mixed and, 98.0 for the dark. (p. 149).”

This is a black-white gap of .43, which is the same as the black-white gap in the Cayman Islands. However the unspecified method Lynn uses to calculate these scores is questionable, given the data reported. I doubt these are meaningful numbers.

Manley, 1963

In the late 1950s thousands of Jamaican children, ages 9-12, took the Moray House Intelligence Test as part of the Common Entrance Exam for secondary school. Children in the top 10% were awarded student aid and scholarships to cover school fees. D.S. Manley (1963) looked at a random sample of 1,730 children from the larger pool of 17,522 who took the examination in 1959.

The Moray House IQ of these academically-oriented school children was 87.3.

This study also provides IQ scores for different demographics. IQ differences are reported for the different parishes of Jamaica. IQ was about 6 points lower in rural areas, than it was in urban areas. Girls scored fractionally higher than the boys. IQ scores are reported for 6 different social classes. Children from the most elite families had an average IQ of 105.4, while children from the least elite families (“unskilled workers”) had an IQ of 82.1).

Intelligence and Cultural Environment (Vernon, 1969 )

In 1963 Philip Vernon tested 50 average Jamaican boys, age 11, on 16 different cognitive ability tests. Their 90th, 50th, and 10th percentile IQ scores are shown below:

Table II: IQ on 16 tests

Table from Vernon, 1969, p. 236

The average IQ score from the 16 tests is 84.

Some of these tests are weaker measures of IQ than others, so rather than making a straightforward average, we can also weight each test according to its relative g loading within this sample (reported in Vernon, 1965 ). IQ scores are lower on the tests that are more strongly correlated with the g factor (e.g. Matrices and Kohs Blocks).

When the various tests are weighted by g, the average IQ is 79.

Hertzig et al., 1972

In this study, 71 children that were severely malnourished in their first two years of life were tested with the Wechsler Intelligence Scale for Children (WISC) when they were 6-10 years old. 34 of their siblings were given the same test. An additional control group of 71 healthy children from the same schools were also tested. The Wechsler test was standardized in 1949. This requires a -7 point adjustment for 22 years of norm inflation.

The malnourished group had a full-scale IQ of 51, the siblings had an IQ of 55, and the control group had an IQ of 59.

Richardson (1976 ) further sub-divided this sample into a disadvantaged group that was malnourished, small at follow-up, and from an unfavorable social background (N=14), and an advantaged group that was not malnourished, tall at follow-up, and from a favorable social background (N=19).

The WISC IQ of the advantaged sub-sample was 62, while the WISC IQ of the disadvantaged sub-sample was 42.

Grantham-McGregor et al., 1991

This paper reports the initial test results from “The Jamaican Study,” which began in 1975. Door to door interviews were conducted in the poor areas of Kingston to locate all the 1-2 year old children that were 2 standard deviations below the US age-norms for height. These 129 stunted children were then divided into four different intervention groups and followed over time. A control group of non-stunted children was also included by selecting infants that were 1 SD above the US height norms (N = 32).

In this paper, Peabody Picture Vocabulary Test results are reported for the primary caretaker of each child. The parents of the stunted children that were given no intervention (N = 33) had an IQ of 79.1, while the parents of the non-stunted children had an IQ of 93.7. (Flynn adjustment = -3 points for 1965 test norms). As a general practice, I’m not going to record the IQ scores from intervention groups, because they add noise to the dataset.

Hamilton, 1981

This is a study of 16 year old A-Level test takers (i.e. Jamaican high school students preparing to attend college). In 1977 the reasoning scale of the Differential Aptitude Test was administered to 140 juniors. The British norms for this test were also collected in 1977 (Hodgkiss, 1979), so the scores require no Flynn adjustment. The mean score was 33.4, which translates to an IQ of 95.

Abstract reasoning scores were significantly correlated with early educational quality (.29), high school quality (.34), and achievement motivation (.22), but unusually, not with socioeconomic status or A-level exam performance.

Persaud, 1982 /1987

Two studies of Jamaican University students were performed with an abridged, Swedish version of the Raven Progressive Matrices. I do not have the comparison norms for this test, which probably only exist in a lost manual at an obscure Swedish library. However, one of the studies (Persaud, 1987) does permit us to compare the scores of different adult age groups on the same test. This can give us a rough idea about the speed of the Flynn Effect in Jamaica. Scores are reported separately for men and women, as well as four different age categories (17-51). If we consolidate the age groups we get a group that was roughly born in the early 1950s, and a group that was roughly born in the late 1960s. The difference between the younger and the older group is about .80 SD. This suggests IQ scores were rising about 5-6 points per decade over the same period, which is about twice the rate that IQ scores have been increasing in Western nations.

Richards, 1988

This is an unusual study with one of the highest IQ scores for a Jamaican sample. The author administered the Draw-a-Man test to 13-year-old Kingston school children (N =43) and a matched group of US school children. The samples are reportedly matched for neighborhood socioeconomic status; so this is an elite Jamaican sample. The Kingston students, for example, were already prepared for the A-Levels. The author reports the deviation IQs from the 1963 DAM norms, which requires a 7 point adjustment. The American sample exhibits an IQ of 88 and the Jamaican sample exhibits an IQ of 98.

Race is not reported for either sample, so the US comparison group could plausibly be African American.

Simeon & Grantham-McGregor, 1989

This study compared three groups of children, ages 9-11: a group that had been hospitalized for malnutrition in the first two years of life, and stunted and non-stunted groups selected from poor schools near the hospital (N=90, or 30 for each group).

The malnourished group exhibited a Peabody Picture Vocabulary Test IQ of 60 (Flynn correction = 2 points), and the stunted group exhibited a PPVT IQ of 61.

The PPVT IQ of the healthy children was 70.

Grantham-McGregor et al., 1994

This is another report for the so-called “Jamaican Study”, which began in 1975. But this is a different sample of children, which I will call Jamaican Study sample II. These are 35 children who were originally hospitalized for malnutrition in their first two years of life. They were divided into an intervention group, which underwent a Head Start-like assistance program for 3 years, and a non-intervention control group. An additional control group of 19 children hospitalized for reasons unrelated to malnutrition was also followed.

At age 16, full scale WISC IQ was 56 for the nonintervention group. WISC IQ for the nonmalnourished control group was 74.

There are also PPVT scores for the adults and the children, but these are reported as raw scores, and I don’t currently have access to the conversion norms.

Knight et al., 1995

This is a comparison of 60 teens (ages 15-18) with sickle cell anemia, and a matched control group without this genetic blood disease. Those under 18 were tested with the WISC-R, while the 18-year-olds were tested with the Wechsler Adult Intelligence Scale-Revised (WAIS-R). The WISC-R IQ of the sickle cell group (N=28) was 66.3 (Flynn correction = -6 points). The WAIS-R IQ of the sickle cell group (N=32) was 77.8 (FC=-3 points).

The WISC-R IQ of the healthy group (N=19) was 67.5, while the WAIS-R IQ of the healthy group (N=41) was 86.2.

Nokes et al., 1992

This study compares the intelligence of children (ages 9-12) with whipworm infection and a matched group of healthy controls. Stool samples were taken from all children ages 9-12 from three schools in Mandeville, Jamaica. Of the 593 children, 69% had a light parasite burden, 17.5% had a moderate to heavy parasite burden, and 13.5% had no indications of infection. Those with moderate to heavy infection were divided into a treatment and a placebo group, and the uninfected group was used as a control.

The average Colored Progressive Matrices IQ for the placebo group was 66 (N= 41). The IQ for the healthy control group was 79 (N=56).

Grantham-McGregor et al., 1997

This is another report from the “Jamaican Study” team. There are several methodologically similar and distinct longitudinal studies going on with this team, all referred to as “The Jamaica Study”, and their samples are not always carefully differentiated. Grantham-McGregor et al. (1991) reported on Jamaica Study Sample I: a group of stunted 1-2 year old infants selected by door-to-door survey in 1975. Grantham-McGregor et al. (1994) reported on Jamaica Study Sample II: a group of malnourished 1-2 year old infants selected from a hospital in 1975. This current study reports on Jamaica Study Sample III: a group of stunted 1-2 year old infants selected by door-to-door survey in 1986.

Intelligence test data is reported for both the caretakers and their children when the sample is around 8 years old (about 1993). The stunted children are divided into four groups: a nonintervention group (N =32), a stimulated group (N =29), a supplemented group (N=30), and a group that is both stimulated and supplemented (N=31). A fifth group of healthy, non-stunted children is also included (N=84). I will report the scores for the two non-intervention groups.

The PPVT IQ of the 32 “nonintervention” caretakers was 75.4 (Flynn correction of -8 points for 1965 test norms), and the PPVT IQ of the 84 “non-stunted” caretakers was 85.

The children were tested on three tests: The PPVT, The Stanford-Binet Intelligence Scale, and the Colored Progressive Matrices. The SBIS norms are from 1974, and require a -8 point adjustment. I’ve used the 1979 CPM-SPM conversion norms for expediency, and this requires a -4 point adjustment. The PPVT scores are unusually low, and it’s probable that they are reporting the raw scores instead of the deviation IQs. I do not have access to the conversion norms, so I can’t include this data.

The SBIS IQ of the 32 nonintervention children was 68. Their CPM IQ was 71.

The SBIS IQ of the 84 nonstunted children was 77. Their CPM IQ was 80.

Averaging these two tests: the IQ of the unhealthy children was 70, and the average IQ of the healthy children was 79.

Callender, 1998

This is an intelligence test study of 7-11 year old children who had suffered from whipworm-associated dysentery four years earlier, as well as a healthy control group. The children were tested with the Stanford-Binet Intelligence Scale, the Peabody Picture Vocabulary Test, the Colored Progressive Matrices, and the Wide Range Achievement Test.

The test administration was around 1994, and this requires a -6 point adjustment for the SBIS, and a -5 point adjustment for the CPM. I do not have current access to the WRAT and PPVT norms, so I can’t convert the raw scores for these tests into deviation IQs.

The SBIS IQ of the 18 dysentery children was 61.5. Their CPM IQ was 60.

The SBIS IQ of the 18 healthy control children was 74. Their CPM IQ was 75.

Walker et al., 2000

This is the longitudinal follow-up of the Jamaica Study Sample III, when they were 12 years old. This would be around 1997.

The caretakers’ PPVT scores were converted on 1981 norms and require a -5 point adjustment. The PPVT IQ of the 31 “nonintervention” caretakers was 78, and the PPVT IQ of the 80 “nonstunted” caretakers was 88.

The children were tested on the WISC-R, the CPM, and the PPVT. Their WISC conversions are from 1974 and require a -5 point Flynn adjustment, my CPM conversions are from 1979 and require a -6 point adjustment, the PPVT norms were 1981 and require a -5 point adjustment.

The WISC-R IQ of the nonintervention group (N=31) was 58. Their CPM IQ was 59. Their PPVT IQ was 60.

The WISC-R IQ of the nonstunted group (N=80) was 73. Their CPM IQ was 66. Their PPVT IQ was 74.

Averaging these three tests: the IQ of the unhealthy children was 59, and the average IQ of the healthy children was 71.

Meeks et al., 2007

This is a study of risk factors in aggression among 12 year old boys from Kingston, Jamaica. Teachers at 10 schools rated all their students on an aggression survey that asked questions about cursing, temper, fighting, etc. 101 of the most aggressive, and 101 of the least aggressive boys were chosen for study. The boys were given the Peabody Picture Vocabulary Test in 1998, and were scored on the 1981 norms, requiring a -5 point adjustment.

The PPVT IQ of the aggressive boys was 65, and the PPVT IQ of the non-aggressive boys was 71.

Samms-Vaughan, 2005

This might be the best IQ study in the Jamaica dataset, in that it is recent, reasonably representative, and used a probability sampling technique to locate random children from six of Jamaica’s 16 parishes.

143 children were tested with the McCarthy Scale of Children’s Abilities, Raven’s Colored Progressive Matrices, the Peabody Picture Vocabulary Test, and the Wide Range Achievement Test. The data was collected in 1999, and this requires a -8 point adjustment for the McCarthy scores, and a -6 point adjustment for the CPM scores. I do not have current access to the WRAT and PPVT norms, so I can’t convert the reported scores for these tests into deviation IQs.

The McCarthy IQ was 78. The CPM IQ was 84. The average IQ from these two tests is 81.

IQ scores were also reported by socioeconomic status. The McCarthy IQ of children in the lowest socioeconomic group was 71. The McCarthy IQ of children in the highest socioeconomic group was 90. The gradient was less steep for the CPM: the lowest socioeconomic group had an IQ of 84, and the highest had an IQ of 86. The Raven test was not significantly correlated with any of their socioeconomic variables, while the other three tests were only very weakly associated with socio-economic variables.

Wachs et al., 2007

This is a study of mothers with children who either had a full-term low birth weight or a full-term normal birth weight.

The mothers were administered the PPVT in about 2001. The reported PPVT scores were converted on 1981 norms, requiring a -6 point adjustment.

This leaves us an IQ of 86.2 for the mothers of low birth weight children (N=121), and an IQ of 95.1 for the mothers of normal birth weight children (N=80).

Walker et al., 2007

This is a longitudinal follow-up of 186 children who have been tracked from the womb to study the origins of adult diseases. Their mothers were originally recruited from the antenatal clinic if they were free from illness or genetic abnormalities.

The 6-8 year old children were given the PPVT and the CPM in 2002. This requires a -7 point adjustment for the CPM. I don’t currently have access to the PPVT norms.

The CPM IQ of these children was 84.

Raven intelligence scores at age 8 were associated with in utero head circumference measured at 14 weeks (r=.17). Raven scores were the only significant association with fetal growth.

Additionally, no significant sex differences were found on the IQ tests.

Walker et al., 2005

The Jamaica Study Sample III was previously IQ tested at age 8 (Grantham-McGregor et al., 1997), and at age 12 (Walker et al., 2000). In this study the same sample is tested again as young adults (age 18).

In 2003 the teens were tested with the Wechsler Adult Intelligence Scale, the Standard Progressive Matrices, and the PPVT. The WAIS scores are not reported, and I do not have access to conversion norms for the PPVT scores. The Standard Progressive Matrices scores require a -7 point adjustment, and this gives the intervention group (N=27) an adult SPM IQ of 58 and the nonstunted group an adult SPM IQ of 67 (N=64).

Walker et al., 2010

This is a continuation of the birth weight study reported above (Wachs et al. 2007). The children and their caregivers were given IQ tests in 2005-2006 when the children were, on average, 6.8 year old. The caregivers were given the PPVT, which was normed in 1981 and requires a -8 point adjustment. The low birth weight group is divided into an intervention and a nonintervention group.

Caregivers (mostly mothers) of low birth weight children had a PPVT IQ of 82 (N=54). Caregivers of normal birth weight children had a PPVT IQ of 97 (N=73).

The children were given the Wechsler Preschool and Primary Test of Intelligence. This test was normed in 2002 and requires a -1 point adjustment. They were also given the PPVT, but I don’t have access to the conversion norms.

The low birth weight group had a WPPSI IQ of 78, and the normal birth weight group had a WPPSI IQ of 81.

Ward, 2012

This is a recent book chapter with Wechlser Adult Intelligence Scale-III norm data for Jamaica, collected in 2001-2003 (N=151). I do not currently have access to WAIS-III norms for the US or the UK, so I can’t convert these norms into comparative deviation IQs. But this is the only reference with IQ test standardization data for Jamaica. If you have access to WAIS-III norms, please send me an email. (⇠ highlight, don’t click)

 
 
       SECTION II: TABULATED IQ DATA FROM 20 REFERENCES       
 
 

Table III features the data summarized above; intelligence test scores are presented for 43 different groups of Jamaicans. The studies are ordered chronologically by the date they were administered (“Admin”). The samples are also coded for normalcy:

A = Average: these samples are reasonably representative of the population. This includes most control groups and other healthy people selected at random from any region.

SA = Slightly Advantaged: these are samples with some reported beneficial quality, and no reported disadvantage.

MA = Mostly Advantaged: these are samples that are highly advantaged, e.g. college students, engineers, rich children.

SD = Slightly Disadvantaged: these are samples with some negative quality, and no reported advantage. The parents and siblings of malnourished children, for example, are part of a below average environment.

MD = Mostly Disadvantaged: these are samples that are highly disadvantaged. This includes people selected for health problems or other misfortunes, e.g. prisoners, street children.

Please note that separate samples are always divided by empty rows. Contiguous rows represent different test scores from the same study sample. These undivided samples are always followed by a row with the (average) score from two or more tests.
 

Table III: IQ test scores in Jamaica

Admin Sample Age N Test IQ Reference
_
1926 A 10-40 586 DAM 58 Davenport, 1929
1926 A 10-40 202 Alpha 96
(68)
_
_
1959 MA 9-12 1730 MH 87 Manley, 1963
_
_
1963 A 11 50 Various 79 Vernon, 1969
_
_
1971 MD 6-10 71 WISC 51 Hertzig, 1972
_
1971 SD 6-10 34 WISC 55
_
1971 A 6-10 71 WISC 59
_
_
1975 A 26 32 PPVT 94 Grantham-McGregor, 1991
_
1975 SD 22 33 PPVT 79
_
_
_
1977 MA 16 140 DAT 95 Hamilton, 1981
_
_
1985 MA 13 43 DAM 98 Richards, 1988
_
_
1988 MD 9-11 30 PPVT 60 Simeon, 1989
_
1988 MD 9-11 30 PPVT 61
_
1988 A 9-11 30 PPVT 70
_
_
1989 MD 16 16 WISC 56 Grantham-McGregor, 1994
_
1989 A 16 19 PPVT 74
_
_
1991 MD 15-17 28 WISC-R 66 Knight, 1995
_
1991 MD 18 32 WAIS-R 78
_
1991 A 15-17 19 WISC-R 68
_
1991 A 18 41 WAIS-R 86
_
_
1991 MD 9-12 41 CPM 66 Nokes, 1992
_
1991 SA 9-12 56 CPM 76
_
_
1993 SD 30 32 PPVT 75 Grantham-McGregor, 1997
_
1993 A 32 84 PPVT 85
_
1993 MD 8 32 SBIS 68
1993 MD 8 32 CPM 71
(70)
_
1993 A 8 84 SBIS 77
1993 A 8 84 CPM 80
(79)
_
_
1994 MD 7-11 17 SBIS 62 Callender, 1990
1994 MD 7-11 17 CPM 60
(61)
_
1994 A 7-11 17 SBIS 74
1994 A 7-11 17 CPM 75
(75)
_
_
1997 SD 34 31 PPVT 78 Walker, 2000
_
1997 A 38 80 PPVT 88
_
1997 MD 11-12 31 WISC-R 58
1997 MD 11-12 31 CPM 59
1997 MD 11-12 31 PPVT 60
(59)
_
1997 A 11-12 80 WISC-R 73
1997 A 11-12 80 CPM 66
1997 A 11-12 80 PPVT 74
(71)
_
_
1999 SD 12 101 PPVT 65 Meeks, 2007
_
1999 SA 12 101 PPVT 71
_
_
1999 A 6 143 CPM 84 Samms-Vaughan, 2005
1999 A 6 143 McCarthy 78
(81)
_
_
2001 SD 14-45 121 PPVT 86 Wachs, 2007
_
2001 A 15-44 80 PPVT 95
_
_
2002 A 6-8 186 CPM 84 Walker, 2007
_
_
2003 MD 18 27 SPM 58 Walker, 2005
_
2003 A 18 64 SPM 67
_
_
2006 SD Adult 54 PPVT 82 Walker, 2010
_
2006 A Adult 73 PPVT 97
_
2006 SD 7 54 PPVT 78
_
2006 A 7 73 PPVT 81
_


 
 
                    SECTION III: LYNN & THE JAMAICAN IQ                    
 
 

Table IV is a list of all the references Lynn has previously given for IQ in Jamaica, as well as the deviation IQs he has reported for those references.

Table IV: Lynn’s citation history for Jamaican IQ

Lynn’s two most frequent citations for this nation have been Manley (1963) and Vernon (1969), which have both been cited five times since his first review of international intelligence studies in 1978.

Manley (1963 ) provides, by far, the largest sample size of any IQ study for Jamaica, so it’s important to clear up errors. Lynn (1991) cited this study in his first systematic review article. He reported an IQ of 72. Lynn’s numbers and this citation were subsequently reprinted in J.P. Rushton’s Race Evolution, and Behavior (1995, p. 136). This was the only study listed for Jamaica in IQ & the Wealth of Nations (2002), and it has been included as a reference for Jamaica in every follow-up review, except the latest book (2012), which contains numerous omissions. In all of these reviews, the IQ for this study is listed as 72. However, the paper explicitly reports an IQ of 87.3 (p. 54). This is fully 1 standard deviation higher than what Lynn has reported for the last 22 years.

Presumably, Lynn was extrapolating from Manley’s statement that “a large number of scores are “bunched” together at the level 70-74 (p. 55)”. But Manley makes it clear that this is because of a floor effect: the lowest possible IQ on this test is about 70, which represents basically no items answered correctly. The bell curve graph next to this statement also makes it clear that most of the test-takers scored in the 80s.

Despite this frustrating inaccuracy, Lynn’s made-up number probably captures the contemporaneous IQ of the nation better than the actual scores, since these children were an advantaged sub-sample of the larger population. For example, Philip Vernon (1969) wrote: “… a good deal is known of the distribution of abilities among Jamaican school children (cf. Manley, op. cit.). However, barely one fifth of all children eligible by age take these tests. From their results one can estimate that the top five per cent of the population obtain [Moray House] quotients of 100 and over; and then by extrapolation that the population median is roughly 75. (p. 168).”

Lynn first summarized Vernon’s study of Jamaican boys in his 1978 chapter on race and intelligence. He noted that the tests showed an IQ in the “low 80s”. However, when he cited this study again in his 1991 review he listed the IQ as 75, and this is the IQ he has reported for this study in his periodical reviews since IQ & Global Inequality (2006). (This study is also missing from the latest book).

But the average of the 16 IQ tests administered by Vernon is, in fact, 84. (see table II)

How did Lynn come up with 75? Flynn corrections and other idiosyncratic score adjustments are typically reported (cf. my own estimate of 79). Vernon does estimate an IQ of 75 for Jamaica in this book, in the very text I just quoted, but this number has absolutely nothing to do with the 50 boys that he himself tested. Presumably, this is where the number comes from, and Lynn has been reporting this number for 22 years by mistake.

Lynn started using six references for Jamaica during 2006. This includes Hertzig et al. (1972), which was also used in his 1991 review article. The IQ was listed as 66 in 1991, which is the exact number that Hertzig reports. Lynn’s updated value is 60, which is a reasonable Flynn adjustment.

Lynn also adds a study by Bagley et al. (1983) and lists an IQ of 75. Bagley’s book chapter reports data for the Children’s Embedded Figures Test from a sample in rural Jamaica. I did not add this study to my own dataset because the CEFT is a measure of “cognitive style”, which psychologists differentiate from cognitive ability. I believe I’ve seen this test correlate with g in other studies, and I may include it as a proxy for IQ later on for certain primitive groups with limited data. But I don’t think it should be included as IQ data for Jamaicans.

Lynn cites Simeon & Grantham-McGregor (1989) and reports the correct data from the paper, with a minor Flynn adjustment.

The final update was two samples from Grantham-McGregor et al. (1994). His IQ for the adult sample is listed as 60, which is just how the number appears in the paper. Unfortunately, this number is not a deviation IQ at all, but a PPVT raw score!

His IQ for the child sample from the same study is an average of the scores from the malnourished intervention and nonintervention samples together with the healthy sample. This boosts the sample size a little bit, but damages the integrity of the estimate. In the only two papers he reports on correctly—Hertzig et al. (1972) and Simeon & Grantham-McGregor (1989)—he included only the data from the healthy control children. Why the inconsistency?

Further, Lynn’s average of the 7 numbers is wrong. In IQ & Global Inequality (2006) the median of these 7 studies is (correctly) reported as 71 (p. 303), but in Race Differences in Intelligence (2006), published the same year, and listing the exact same 7 samples, the median is reported as 67 (p. 41). This mistake is repeated in The Global Bell Curve (2008, p. 140). Intelligence: A Unifying Construct… (2012) simply omits four references, presumably to force the data into compliance with the faulty 67 median (p. 403).

So not only were most of the numbers from Jamaican studies reported incorrectly (always towards much lower IQs), but the 7 numbers weren’t even averaged correctly, and samples have been arbitrarily included and removed, presumably (in the case of the latest book, at least) for no logical reason than to fit the data to predetermined estimates.

An Updated Estimate of Jamaican IQ

Table III provides data for making a new, more accurate, more transparent estimate of Jamaican national IQ. There is no One True Way to do this, and different methods will have different limitations. An average of all the scores will include many samples that were deliberately chosen for health problems (The median IQ of all the samples in the table is 75). I’ve rated the samples for quality to help limit the influence of less representative test groups. 19 of the 43 samples were disadvantaged, 5 of the samples were advantaged, and 19 were reasonably representative. The median IQ of the disadvantaged samples was 66, while the median IQ of the advantaged samples was 87.

The median of 19 reasonably representative samples gives us an IQ of 79 for Jamaica. This is also consistent with Samms-Vaughan (2005 ), one of the more recent and methodologically robust studies, which shows an average IQ of about 81.

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3 thoughts on “HVGIQ: Jamaica

  1. Wow, lots of great work here.

    Any trend over time in the representative scores?

    Here’s a question. I’ve noticed a stereotype among West Indians that Barbadians are seen as best educated, perhaps followed by Trinidadians (leaving out the big Indian population), with Jamaicans lagging. Is that stereotype of higher IQs in Barbados confirmed in the literature?

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  3. Pingback: Welcome Readers from Portugal! | JayMan's Blog

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