Twin–Singleton Differences in Cognitive Abilities in a Sample of Africans in Nigeria

Abstract

Recent studies comparing cognitive abilities between contemporary twins and singletons in developed countries have suggested that twin deficits in cognitive abilities no longer exist. We examined cognitive abilities in a sample of twins and singletons born recently in Nigeria to determine whether recent findings can be replicated in developing countries. Our sample consisted of 413 pairs of twins and 280 singletons collected from over 45 public schools in Abuja and its neighboring states in Nigeria. The ages of twins and singletons ranged from 9 to 20 years with a mean (SD) of 14.6 years (2.2 years) for twins and 16.1 years (1.8 years) for singletons. Zygosity of the same-sex twins was determined by analysis of 16 deoxyribonucleic acid markers. We asked participants to complete a questionnaire booklet that included Standard Progressive Matrices-Plus Version (SPM+), Mill-Hill Vocabulary Scale (MHV), Family Assets Questionnaire, and demographic questions. The data were corrected for sex and age and then analyzed using maximum likelihood model-fitting analysis. Although twins and singletons were comparable in family social class indicators, singletons did better than twins across all the tests (d = 0.10 to 0.35). The average of d for SPM+ total [0.32; equivalent to 4.8 Intelligence Quotient (IQ) points] and d for MHV (0.24; equivalent to 3.6 IQ points) was 0.28 (equivalent to 4.2 IQ points), similar to the twin-singleton gap found in old cohorts in developed countries. We speculate that malnutrition, poor health, and educational systems in Nigeria may explain the persistence of twin deficits in cognitive abilities found in our sample.

Neurodevelopmental Outcomes of Twins Compared With Singleton Children: A Systematic Review

“…Singletons had significantly higher academic ratings than twins in the other two studies (de Zeeuw et al, 2012;Hur & Lynn, 2013). de Zeeuw et al (2012) studied twins and their non-twin siblings registered with the Netherlands Twin Register and rated their proficiency in arithmetic, language, reading, physical education, and a national educational achievement test score (CITO).…”

“…We examined the remaining eight studies and four of them were prospective follow-up studies (Bodeau-Livinec et al, 2013; de Zeeuw et al, 2012; Eras et al, 2013; Gnanendran et al, 2015), three were cross-sectional studies (Hjern et al, 2012; Hur & Lynn, 2013; Raz et al, 2016), while one was a randomized controlled trial (Manuck et al, 2014).…”

“…We identified eight articles published between 2011 and March 2017 that compared neurodevelopmental outcomes of twins and singletons. Of the eight articles, five showed that there was no significant difference in neurodevelopmental outcomes between twins and singletons (Bodeau-Livinec et al, 2013; Eras et al, 2013; Gnanendran et al, 2015; Hjern et al, 2012; Manuck et al, 2014), while two showed that singletons had better academic outcomes than twins (de Zeeuw et al, 2012; Hur & Lynn, 2013). We did not conduct a meta-analysis because eight articles were too few and the studies used different study populations and instruments to measure neurodevelopmental outcomes.…”

“…Two studies of pre-term singletons and twins adjusted for gestational age but not birth weight and did not find any difference in neurodevelopmental outcomes (Bodeau-Livinec et al, 2010; Manuck et al, 2014). Three studies did not adjust for gestational age or birth weight (Eras et al, 2013; Hjern et al, 2012; Hur & Lynn, 2013) and one showed no difference (Eras et al, 2013), while two showed a better neurodevelopmental outcome among singletons (Hjern et al, 2012; Hur & Lynn, 2013). Comparison of these results is challenging because of differences in the study population, study designs, number of covariates measured, and how neurobehavioral outcomes were measured.…”

The Nigerian Twin and Sibling Registry: An Update

“…As indicated, singletons outperformed twins on Standard Progressive Matrices plus (Raven, 2008) and the Mill-Hill Vocabulary test (Raven, 2008). The average difference between twins and singletons was equivalent to 4.2 IQ points (d = 0.28; Hur & Lynn, 2013). These findings were similar to the twin-singleton gap found in earlier Western cohorts, but in contrast to the results from recent studies using contemporary twins in developed countries where no or a very small twin penalty is found (e.g., Christensen et al, 2006;Posthuma et al, 2000).…”