Van Ijzendoorn, M. H., Juffer, F., & Poelhuis, C. W. K. (2005). Adoption and cognitive development: a meta-analytic comparison of adopted and nonadopted children’s IQ and school performance. Psychological bulletin, 131(2), 301.

It turns out that someone already did a meta-analysis of adoption studies and cognitive ability. It does not solely include cross-country transracial, but it does include some. They report both country of origin and country of adoption, so it is fairly easy to find the studies that one wants to take a closer look at. It is fairly inclusive in what counts as cognitive development, e.g. school results and language tests count, as well as regular IQ tests. They report standardized differences (d), so results are easy to understand.

They do not present aggregated results by country of origin however, so one would have to do that oneself. I haven’t done it (yet?), but the method to do so is this:

  1. Obtain the country IQs for all countries in the study. These are readily available from Lynn & Vanhanen (2012) or in the international megadataset.
  2. Score all the outcomes by using the adoptive country’s IQ. E.g. if the US has a score of 97, and Koreans adopted to that country have get a d score of .16 in “School results” as they do in the first study listed, then this corresponds to a school IQ performance of 97 – 2.4 = 94.6. Note that this assumes that the comparison sample is unselected (not smarter than average). This is likely false because adoptive parents tend to be higher social class and presumably smarter, so they would send their (adoptive) children to above average schools. Also be careful about norm comparisons because they often use older norms and the Flynn effect thus results in higher IQ scores for the adoptees.
  3. Copy relevant study characteristics from the table, e.g. comparison group, sample sizes, age of assessment and type of outcome (school, language, IQ, etc.).
  4. Repeat step (2-3) for all studies.
  5. BONUS: Look for additional studies. Do this by, a) contacting the authors of recent papers and the meta-analysis, b) search for more using Google Scholar/other academic search engine, c) look thru the studies that cite the included studies for more relevant studies.
  6. BONUS: Get someone else to independently repeat steps (2-3) for the same studies. This checks interrater consistency.
  7. Aggregate results (weighted mean of various kinds).
  8. Correlate aggregated results with origin countries’ IQs to check for spatial transferability, a prediction of genetic models.
  9. Do regression analyses to see of study characteristics predict outcomes.
  10. Write it up and submit to Open Differential Psychology (good guy) or Intelligence (career-focused bad guy). Write up to Winnower or Human Varieties if lazy or too busy.

The main results table

More likely, you are too lazy to do the above, but you want to sneak peak at the results. Here’s the main table from the paper.

Study Country/region of study Country/region of child’s origin Age at assessment (years) Age at adoption (months) N Adoption N Comparison Preadoption status Comparison group Outcome (d)
Andresen (1992) Norway Korea 12-18 12-24 135 135 Not reported Classmates School results 0.16 Language 0.09
Benson et al. (1994) United States United States 12-18 < 15 881 Norm Not reported Norm group School results —0.36
Berg-Kelly & Eriksson (1997) Sweden Korea/India 12-18 < 12 125 9204 Not reported General population School results 0.03 f/—0.04 m Language —0.02 f/—0.05 m
Bohman (1970) Sweden Not reported 12-18 < 12 160 1819 Not reported Classmates School results 0.09 f/0.07 m Language 0.02 f/—0.02 m Learning problems 0.00
Brodzinsky et al. (1984) United States United States 4-12 < 12 130 130 Not reported General population School competence 0.62 f/0.51 m
Brodzinsky & Steiger (1991) United States Not reported 9-19 441 6753 Not reported Population % School failure 0.76
Bunjes & de Vries (1988) Netherlands Korea
India
Bangladesh
Colombia
4-12 12-24 118 236 Not reported Classmates School results 0.24 Language 0.22
Castle et al. (2000) England England 4-12 < 12 52 Norm Not reported Standardized scores School results —0.47, IQ 0.47
Clark & Hanisee (1982) United States Vietnam
Korea
Cambodia
Thailand
0-4 12-24 25 Norm Not reported Standardized scores IQ -2.42
Colombo et al. (1992) Chile Chile 4-12 0-12 16 ii Undernutrition Biological siblings IQ -1.16
Cook et al. (1997) Europe Not reported 4-8 12-24 131 125 Not reported General population School competence 0.56 f/0.16 m
Dalen (2001) Norway Korea
Colombia
12-18 0-12 193 193 Not reported Classmates School results 0.47 (Colombia), —0.07 (Korea)
Language 0.43 (Colombia), —0.05 (Korea)
Learning problems 0.50
Dennis (1973) United States Lebanon 2-18 > 24 85 51 Institute Institute children IQ —1.28 (intraracial), —1.36 (transracial)
De Jong (2001) New Zealand Romania/Russia 4-15 12-24 116 Norm Some problems General population School competence 0.65
Duyme (1988) France France 12-18 < 12 87 14951 Not reported General population School results 0.00
Fan et al. (2002) United States United States 12-18 514 17241 Not reported General population School grades —0.02
Feigelman (1997) United States Not reported 8-21 101 6258 Not reported General population Education level —0.03
Fisch et al. (1976) United States United States 4-12 < 12 94 188 No problems General population IQ 0.00
School results 0.50 Language 0.52
Frydman & Lynn
(1989)
Belgium Korea 4-12 12-24 19 Norm Not reported Standardized scores IQ -1.68
Gardner et al. (1961) United States Not reported 12-18 < 12 29 29 Not reported Classmates School achievement 0.09
Geerars et al. (1995) Netherlands Thailand 12-18 < 12 68 Norm Not reported Population % School results 0.19
Hoopes et al. (1970) United States United States 12-18 100 100 1-2 shifts in placement General population IQ 0.12
Hoopes (1982) United States United States 4-12 < 12 260 68 Nothing special General population IQ 0.18
Horn et al. (1979) United States United States 3-26 < 1 469 164 No problems Environment siblings IQ 0.17/0.34/—0.05
W. J. Kim et al. (1992) United States Not reported 12-18 43 43 Not reported General population School results 0.74
W. J. Kim et al. (1999) United States Korea 4-12 < 12 18 9 Nothing special Environment siblings School competence —0.39
Lansford et al. (2001) United States Not reported 12-18 111 200 Not reported General population School grades 0.46
Leahy (1935) United States United States 5-14 < 6 194 194 Not reported General population School grades 0.00 IQ -0.06
Levy-Shiff et al. (1997) Israel Israel
South America
7-13 < 3 5050 Norm
Norm
Not reported Standardized scores IQ -1.10 f/—2.00 m
Lien et al. (1977) United States Korea 12-18 > 24 240 Norm Undernutrition Standardized scores IQ 0.00
Lipman et al. (1992) Canada Not reported 4-16 104 3185 Not reported General population School performance —0.05 f/0.16 m
McGuinness & Pallansch (2000) United States Soviet Union 4-12 > 24 105 1000 Long time in orphanages Norm group School competence 0.46
Moore (1986) United States United States 7-10 12-24 23 Norm Not reported Standardized scores IQ -0.00 f/—1.00 m
Morison & Ell wood (2000) Canada Romania 4-12 12-24 59 35 Orphanages General population IQ 1.45 (combined)
Neiss & Rowe (2000) United States 75% LTnited States 12-18 392 392 Not reported General population IQ 0.08
O’Connor et al. (2000) England Romania 6 0-42 207 Norm Orphanage Standardized scores IQ —0.56 (combined)
Palacios & Sanchez (1996) Spain Spain 4-12 > 24 210 314 Not reported Institute children School competence —0.18
Pinderhughes (1998) United States United States 8-15 24—48 66 33 Older children General population School competence 0.64 (combined)
Plomin & DeEries (1985) United States United States 1 0-5 182 182 Not reported General population IQ 0.14
Priel et al. (2000) Israel 75% Israel 8-12 12-24 50 80 Not reported General population School competence 0.77 f/1.12 m
Rosenwald (1995) Australia 73% Korea Asia
South America
4-16 < 12 283 2583 Not reported General population School performance —0.18
Scarr & Weinberg (1976) United States 88% LTnited States 4-16 <12 176 145 Not reported Environment siblings IQ 0.75 (combined)
Schiff et al. (1978) France France 4-12 <12 32 20 Not reported Biological siblings School results —0.70
Segal (1997) United States United States 4-12 < 12 6 6 Not reported Environment siblings IQ -1.14 IQ 2.67
Sharma et al. (1996) United States 81% United States 12-18 12-24 4682 4682 Not reported General population School results 0.37 (combined)
Sharma et al. (1998) United States United States 12-18 < 12 629 72 Not reported Environment School competence —0.45 f/—0.61 m
Silver (1970) United States Not reported 4-12 < 3 10 70 Not reported General population Learning problems 1.21
Silver (1989) United States Not reported 4-12 39 Perc. Not reported General population Learning problems 1.38
Skodak & Skeels (1949) United States Not reported 12-18 < 6 100 100 Not reported Standardized scores IQ -1.12
Smyer et al. (1998) Sweden Not reported Adults < 12 60 60 Not reported Biological (twin siblings) Education level —0.82
Stams et al. (2000) Netherlands Sri Lanka
Korea
Colombia
4-12 < 6 159 Norm Not reported Standardized scores School results 0.33 IQ -0.34 f/—0.73 m Learning problems —0.05
Teas dale & Owen (1986) Denmark Not reported Adults < 12 302 4578 Not reported General population IQ 0.35
Education level 0.32
Tizard & Hodges (1978) England Not reported 8 > 24 25 14 Not reported Restored children IQ —0.40 (older), —0.62 (younger)
Tsitsikas et al. (1988) Greece Greece 5-6 < 12 72 72 Not reported Classmates IQ 0.64, school performance 0.29 Language 0.30
Verhulst et al. (1990) Netherlands Europe
Korea
Colombia
India
12-18 > 24 2148 933 Not reported General population Perc. special education 0.25 f/0.29 m
Versluis-den Bieman & Verhulst (1995) Netherlands Europe
Korea
Colombia
India
12-18 > 24 1538 Norm Not reported General population School competence 0.28 f/0.41 m
Wattier & Frydman (1985) Belgium Korea 89% 4-12 12-24 28 Norm Not reported Standardized scores IQ -0.06
Westhues & Cohen (1997) Canada Korea 40% India 40% South America 12-18 12-24 134 83 Not reported Environment siblings School performance 0.13
Wickes & Slate (1997) United States Korea > 18 > 36 174 Norm Not reported Norm group School results 0.09 f/0.07 m Language 0.07 f/0.03 m
Winick et al. (1975) United States Korea 4-12 > 24 112 Norm Malnourished Standardized scores School performance 0.00 IQ 0.00
Witmer et al. (1963) United States United States 12-18 < 12 484 484 Nothing special Classmates School performance 0.00 IQ 0.00