Our results for Primary 6 and Secondary 3 end-of-year examinations indicated no consistent pattern of association between prenatal or recent postnatal MeHg hair levels and any outcome, either at the end of primary school or in mid-high school. In all, we studied 12 end-of-year examination scores, six at the end of Primary 6 and six at the end of Secondary 3. We found one association between prenatal MeHg exposure and 11 out of 12 achievement scores, and no association between recent postnatal exposure and 11 out of 12 examination scores. At the end of Primary 6, we found a significant negative association between prenatal exposure and achievement in French and another significant negative association between recent postnatal exposure and Social Studies achievement. These findings may not have statistical or biological meaningfulness given the lack of associations in these content areas at other time points and may have resulted by chance alone. Taken together, they do not indicate any pattern of either adverse or beneficial MeHg effects on end-of-year achievement test scores.
The results of our analysis of the SACMEQ achievement scores from a subgroup of 215 main cohort children differed from the end-of-year examination results. We found no evidence of associations between prenatal MeHg hair level and outcomes in either subject examined. However, we did find significant adverse associations between recent postnatal MeHg hair level and outcomes in both subjects examined in boys only. These results are different from the subjects’ scores on the Primary 6 end-of-year exams and could also have resulted by chance alone. The mean prenatal MeHg exposure for these 215 subjects was similar to that for the overall subgroup. But the mean recent postnatal MeHg exposure level for the 215 subjects was nearly 2 ppm higher than for the overall subgroup. We cannot discern any reason to explain the difference in recent postnatal exposure. The adverse associations with slightly higher exposure on the SAQMEC exams may be significant if they can be replicated in a larger cohort. Previous studies have indicated that 10 ppm in maternal hair may be the lower end of a threshold effect from prenatal exposure (Cox, et al., 1989
; Huang, et al., 2006). There are very limited data on recent postnatal MeHg effects on cognition in children. Recently, Myers and colleagues (2009)
reported some adverse effects of recent postnatal MeHg exposure on some but not all cognitive endpoints. Our findings appear to extend that report to scholastic achievement scores. These findings are sporadic and not consistent, but do suggest that further work is needed on recent postnatal exposure, both as a separate exposure source and as a potential effect modifier to prenatal MeHg exposure.
Our results for prenatal exposure associations are consistent with our previous findings (Davidson, et al., 1998
; Myers, et al., 2003
) with respect to cognitive functions. Cognitive functions such as IQ or memory are expected to individually influence achievement, which they did in this study, but the impact was not related to prenatal MeHg. Cognitive functions were related to socioeconomic and geographic variables such as SES and school district setting (as a proxy for classroom size and teacher attention), which are independently associated with achievement.
The gender effects present in this analysis are intriguing. The analysis of outcomes at the conclusion of Primary 6 indicated evidence of gender effects. In most cases, girls outperformed boys. In almost all cases these results were independent of either prenatal or recent postnatal MeHg exposure. But in one case (French), there was a significant positive association between prenatal MeHg and achievement for girls only. Again, this outcome does not constitute evidence of any pattern of associations between MeHg and achievement. There are a number of reports in the literature that suggest a greater vulnerability to neurotoxic effects on cognition in males (Vahter, et al., 2007
). There may be many reasons for this outcome, including metabolic differences favoring females in excretion, distribution and retention of both organic and inorganic forms of Hg (Thomas, et al., 1986
) and in the activity of antioxidant defenses (Borras et al., 2003
) which some studies have linked to estrogen receptors (Olivieri, et al., 2002
). The consistency of better scores among girls across both ages and most academic subjects reinforces previous reports from the Government of Seychelles that identified pervasive gaps between girls and boys in both Primary and Secondary educational outcomes (Leste, et al., 2004). The reasons for these gaps are unclear and warrant further examination.
The achievement scores at both ages were noticeably low across all subjects. This outcome has been noted in recurrent examination reports produced by the Assessment and Testing section in the Ministry of Education (Benstrong, E., personal communication). There is concern that a number of factors may influence the average scores on end-of-year exams, including teacher competency including the mechanics of teaching and associated subject knowledge, curriculum design, and the correlation between end-of-year test content and what is actually taught in classrooms across the educational system. In particular, administrators in the Ministry of Education have suspected that teacher competency may account in part for the very low Mathematics competency among Seychellois young people (Leste, et al,. 2004). More recent intervention strategies to improve classroom practice of Mathematics teachers in primary schools have led to some improvement in student average scores in some schools (Leste, A., personal communication). The identification and coding of effective teaching practices are important factors to be included in further examination of scholastic achievement.
Achievement outcomes at both ages were directly influenced by covariates that typically would be expected to influence school achievement in general. For example, almost all outcomes were positively correlated with child IQ and with scores on tests of spatial and verbal memory. Cultural variables, including SES and home environment, less consistently affected achievement, perhaps as should be expected for pre-adolescent and adolescent children.
The strengths of this study were that the sub-cohorts for each evaluation were of substantial size, extensive data was available on the individual subjects, and both the end-of-year examinations and the SAQMEC test were standardized. In addition, the SACMEQ examinations were normed across all participating nations, for two endpoints data were available on the subjects’ teachers, and there was no co-exposure to other toxicants. The study also has limitations. Not all of the main cohort children had data available. Norms did not exist for the tests administered at Primary 6 and Secondary 3; consequently, some of the variability in educational achievement could be related to the fact the children were enrolled in multiple schools with teachers who varied in training, ability, and motivation. In addition, the inclusion of measures of cognitive ability in our models could have increased the probability of over control.