An investigation of the difference in measured intelligence between twins and single births*

Abstract

SUMMARYMean V.R. (verbal reasoning) scores recorded in the eleven‐plus examination for Birmingham multiple births in the years 1950‐57 were 95‐7 for 2164 twins and 91‐6 for 33 triplets. The mean for 48,913 single children born in the years 1950‐54 was 100‐1.The low scores of twins are not explained by differences from single births in their distributions by maternal age and birth order or by birth weight and duration of gestation. They are also not accounted for by the increased risks associated with monozygosity (assessed by comparison of like‐ and unlike‐sex twins) or with delivery of the second twin. Taken together these observations, like the previous ones on single births, suggest that variation in experience before and during birth has little influence on measured intelligence and that the explanation of the large difference between twins and single children must be sought in the postnatal environment.There were 148 twins whose co‐twins were stillborn or died within 4 weeks after birth; their mean score was 98‐8, only a little lower than that of single births (99‐5) standardized to the maternal age and birth rank distribution of twins. From this evidence it is concluded that the handicapping of twins, reflected in their low verbal reasoning scores, is due to postnatal rather than prenatal influences.These conclusions are of course based on children who took the eleven‐plus examination and cannot be accepted without reservations for those who did not.

The Cognitive Cost of Being a Twin: Two Whole-Population Surveys

“…In the Collaborative Perinatal Project of the United States, twins raised as singletons scored at the same level as twins, suggesting a prenatal origin to their lower mental scores (Myrianthopoulos et al, 1976). On the other hand, the Birmingham, United Kingdom, study found that in cases where one twin was stillborn or died in the first 4 weeks of life, surviving twins scored almost at singleton levels (Record et al, 1970). They concluded that 'the large differences in weight and duration of gestation between single and multiple births do not result in appreciable score differences ' (p. 19).…”

“…It would be interesting to quantify twin-twin differences in the quality of intrauterine environment and correlate this with mental ability discordances between twin pairs. One possible indicator might be birthweight, which was found to be associated with twin score decrements in some twin studies (Alin Akerman & Fischbein, 1991;Record et al, 1970). Twins have a smaller birthweight, and birthweight is related to intelligence test scores in childhood .…”

Model-fitting approaches to the analysis of human behaviour

“…Since the majority of large twin and sibling studies give consistent results (e.g. Record et al, 1970), the weight now resides with adoption studies with greater attention paid to the estimation of placement effects which lead to overestimation of the familial environment.…”

Twin Specific Risk Factors in Primary School Achievements

“…The estimates of the difference in intelligence range from 5.1 to only 0.5 IQ points in studies from different countries, populations, and birth cohorts. Several studies based on birth cohorts from many years ago found a lower intelligence in twins (Record et al, 1970;Deary, Pattie, Wilson, & Whalley, 2002;Ronalds, de Stavola, & Leon, 2005). However, prenatal and perinatal care has improved in the past decades, which may have reduced this difference in cognitive ability between twins and singletons.…”

“…The question of whether twins differ from singletons in their cognitive abilities due to the risk factors associated with their twin status has been the focus of research for a long time (Hay & O'Brien, 1983;Record, McKeown, & Edwards 1970;Vandenberg, 1984). A recent meta-analysis of studies on differences in intelligence between twins and singletons concluded that, on average, twins seem to have lower IQs than singletons (Voracek & Haubner, 2008).…”