Respondents
Data were collected from 8984 US respondents (4385 women and 4599 men), born between 1980 and 1984, who took part in the 1997 National Longitudinal Survey of Youth (NLSY97).
Measurements
Most respondents (complete data available for 7008 individuals) took 12 Armed Services Vocational Aptitude Battery (ASVAB) tests in 1997–1998: (1) general science; (2) arithmetic reasoning; (3) word knowledge; (4) paragraph comprehension; (5) numerical operations; (6) coding speed; (7) auto information; (8) shop information; (9) mathematical knowledge; (10) mechanical comprehension; (11) electronics information; (12) assembling objects.
On later occasions, the respondents rated to what degree they felt that a number of personality trait items were descriptive of themselves: (1) disorganized, (2) conscientious, (3) undependable, (4) thorough, (5) agreeable, (6) difficult, (7) stubborn, (8) trustful, (9) extraverted/enthusiastic, (10) critical/quarrelsome, (11) dependable/self-disciplined, (12) anxious/easily upset, (13) open/complex, (14) reserved/quiet, (15) sympathetic/warm, (16) disorganized/careless, (17) calm/emotionally stable, (18) conventional/uncreative. Items 1–8 were rated in 2002 on a scale from 1 to 5 (n between 4857 and 4875) while items 9–18 were rated in 2008 on a scale from 1 to 7 (n between 7195 and 7457). The different response scales for items 1–8 and 9–18 were handled by standardizing the scores before calculations.
Based on item content, the 18 personality trait items were initially grouped into the following three composite scores: (a) Dependable, with the items (see above) 1 (reversed), 2, 3 (reversed), 4, 11, and 16 (reversed); (b) Agreeable, with the items 5, 6 (reversed), 7 (reversed), 8, 10 (reversed), and 15; (c) Open, with the items 9, 12 (reversed), 13, 14 (reversed), 17, and 18 (reversed). As item 2 (conscientious) had a very low factor loading (0.167) and decreased the measured homogeneity of the composite, it was removed. This increased the fit of the factor structure (Akaike Information Criterion (AIC) decreased from 313,257 to 299,545). The composites dependable and agreeable correspond quite closely to the Big Five personality traits Conscientiousness and Agreeableness, respectively [11]. However, we chose to name the first composite dependable rather than conscientious as the conscientious item was removed from the composite. The open composite could be seen to correspond to a combination of the Big Five personality traits Openness to Experience, Extraversion, and low Neuroticism. We are aware that this third composite is a bit of a hodgepodge. However, the main objective of the present study was to evaluate if intelligence by personality trait interaction effects on academic achievement tend to be synergistic or compensatory, and this objective should be achievable even if not all of the used personality trait measures are perfectly orderly. In general terms, to estimate if the A by B interaction effect on C is synergistic or compensatory is possible even if it is not as clear what B measures. The homogeneity (as measured by coefficient Omega) was 0.54, 0.55, and 0.52 for the composites dependable, agreeable, and open, respectively.
Credit weighted overall grade point average (GPA) was available, as transcript survey data from high schools, for 6004 respondents. Moreover, in 2017 respondents were asked about their highest academic degree received, with the values: (0) None, n = 515, (1) General educational development, n = 862, (2) High school diploma, n = 2692, (3) Associate/junior college, n = 598, (4) Bachelor’s degree, n = 1352, (5) Master’s degree, n = 540, (6) Professional degree/PhD, n = 149. Data on degree was available for 6708 respondents.
Statistical analyses
Factor scores on the first unrotated factor in an analysis of all 12 ASVAB tests was used as an estimate of the respondents’ general intelligence. The scores on each personality trait item as well as the three composites were standardized and the correlation with intelligence and grades/degree as well as the item by intelligence interaction effect on grades/degree were calculated. The interaction effects were estimated both without and with adjustment for possible quadratic associations. In the latter case the predicted grade/degree is given by:
$$E\left| {gr/de} \right| = b_{0} + b_{1} \times intell + b_{2} \times intell^{2} + b_{3} \times trait + b_{4} \times trait^{2} + b_{5} \times intell \times trait$$
(1)
If the personality trait by intelligence interaction effect on grades/degree tends to be synergistic or complementary, we should see a positive or a negative association between the item-grades/degree correlation and the coefficient for the item by intelligence interaction effect, respectively. The present study included 195 tests of significance, and if wishing to be cautious not to conduct type 1-errors, a conservative Bonferroni correction could be used to set the significance level at 0.05/195 = 0.000256. Analyses were conducted with R 4.0.2 statistical software [12] employing the psych [13] and lavaan [14] packages. Code and dataset are available at https://osf.io/2fsca/.