Comparison of academic performance of twins and singletons in adolescence : follow-up study

Objectives To determine whether twins in recent cohorts show similar academic performance in adolescence to singletons and to test the effect of birth weight on academic performance in twins and singletons. Design Follow-up study. Setting Denmark. Participants All twins (n = 3411) and a 5% random sample of singletons (n = 7796) born in Denmark during 1986-8. Main outcome measures Test scores in ninth grade (age 15 or 16), birth weight, gestational age at birth, parents’ age, and parents’ education. Results Ninth grade test scores were normally distributed, with almost identical mean and standard deviations for twins and singletons (8.02 v 8.02 and 1.05 v 1.06) despite the twins weighing on average 908 g (95% confidence interval 886 to 930 g) less than the singletons at birth. Controlling for birth weight, gestational age at birth, age at test, and parents’ age and education confirmed the similarity of test scores for twins and singletons (difference 0.04, 95% confidence interval − 0.03 to 0.10). A significant, positive association between test score and birth weight was observed in both twins and singletons, but the size of the effect was small: 0.06-0.12 standard deviations for every kilogram increase in birth weight. Conclusions Although older cohorts of twins have been found to have lower mean IQ scores than singletons, twins in recent Danish cohorts show similar academic performance in adolescence to that of singletons. Birth weight has a minimal effect on academic performance in recent cohorts; for twins this effect is best judged relative to what is a normal birth weight for twins and not for singletons

Birth order, gestational age, and risk of delivery related perinatal death in twins: retrospective cohort study

Objective: To determine whether twins born second are at increased risk of perinatal death because of complications during labour and delivery. Design: Retrospective cohort study. Setting: Scotland, 1992 and 1997. Participants: All twin births at or after 24 weeks' gestation, excluding twin pairs in which either twin died before labour or delivery or died during or after labour and delivery because of congenital abnormality, non-immune hydrops, or twin to twin transfusion syndrome. Main outcome measure: Delivery related perinatal deaths (deaths during labour or the neonatal period). Results: Overall, delivery related perinatal deaths were recorded for 23 first twins only and 23 second twins only of 1438 twin pairs born before 36 weeks (preterm) by means other than planned caesarean section (P>0.99). No deaths of first twins and nine deaths of second twins (P=0.004) were recorded among the 2436 twin pairs born at or after 36 weeks (term). Discordance between first and second twins differed significantly in preterm and term births (P=0.007). Seven of nine deaths of second twins at term were due to anoxia during the birth (2.9 (95% confidence interval 1.2 to 5.9) per 1000); five of these deaths were associated with mechanical problems with the second delivery following vaginal delivery of the first twin. No deaths were recorded among 454 second twins delivered at term by planned caesarean section. Conclusions: Second twins born at term are at higher risk than first twins of death due to complications of delivery. Previous studies may not have shown an increased risk because of inadequate categorisation of deaths, lack of statistical power, inappropriate analyses, and pooling of data about preterm births and term births

Is humn mating adventitious or the result of lawful choice? A twin study of mate selection.

Inventory data on a large sample of middle-aged twins and their spouses confirmed that spousal pairs are consistently but weakly similar on traits of personality, interests, talents, and attitudes. We argue, however, that neither the Similarity model of mate selection, nor one of its facets, the Equity model, can account for specific mate choice. We therefore tested the hypothesis that people select their mates using idiosyncratic criteria and that the spouses of monozygotic (MZ) twins should therefore be very similar. When compared to spouses of dizygotic (DZ) twins or even to random pairs of spouses, the spouses of MZ twins failed to show the predicted excess of small intra-spouse differences. We asked 547 of these twins to rate their attitudes toward their cotwin's choices of wardrobe, furnishings, vacations, jobs - and spouses; a similar questionnaire was completed by the spouses of these twins. Both data sets confirm that MZ twins are very similar in most of their choices, more so than DZ twins, but nearly 40% of both MZs and DZs recall that they actually disliked their cotwin's choice of mate at the time that choice was made. Similarly, 30% of the spouses of MZ twins report actually disliking the identical twin of the mate they had recently selected. Our findings suggest that characteristics both of the chooser and the chosen constrain mate selection only weakly. We propose that it is romantic infatuation that commonly determines the final choice from a broad field of potential eligibles and that this phenomenon is inherently random, in the same sense as is imprinting in precocial birds

Research with twins: The concept of emergenesis.

Preliminaty findings from an on-going study of monozygotic twins reared apart (MZA) and data from a larger sample of twins reared together (MZT and DZT), indicate a surprisingly strong influence of genetic variation on aptitudes, psychophysiological characteristics, personality traits and even dimensions of attitude and interest. For some of these variables, MZT and MZA twins show high intra-class correlations while DZT twins are no more similar than pairs of unrelated persons. It is suggested that such traits are “emergenic,” i.e., that they are determined by the interaction--rather than the sum--of genetic influences. Emergenic traits, although perhaps strongly genetic, will not tend to run in families and for this reason have been neglected by students of behavior genetics. For this and several other listed reasons, wider use of twins in psychological research is strongly recommended

Speaking to twin children: evidence against the "impoverishment" thesis

It is often claimed that parents’ talk to twins is less rich than talk to singletons and that this delays their language development. This case study suggests that talk to twins need not be impoverished. We identify highly sophisticated ways in which a mother responds to her 4-year-old twin children, both individually and jointly, as a way of ensuring an inclusive interactional environment. She uses gaze to demonstrate concurrent recipiency in response to simultaneous competition for attention from both children, and we see how the twins constantly monitor the ongoing interaction in order to appropriately position their own contributions to talk. In conclusion, we argue for the need to take twins’ interactional abilities into account when drawing linguistic comparisons between twins and singletons. Data are in Australian English

Twins in Mesoamerica as a Symbol of Contrasting Duality

In cultures across the Mesoamerican subcontinent, there are examples in the archaeological and ethnographic record of myths concerned with twins and sibling pairs (Minneci, 1999). Twins in particular are seen as potent mythic pairs; however, both twins and other pairs of siblings are utilized in iconography and mythology to represent connected opposites. The idea of contrasting duality, where the two halves of each pairing are independent and yet connected to one another, is vital to the Mesoamerican way of thinking. Cultures such as the Maya and the Aztec used spouses and siblings, twins in particular, to embody contrasting counterparts of duality and completion, an idea that structures the worldviews of these two cultures. For the Maya, the most prominent example of mythic siblings would be the Hero Twins in the Popol Vuh. Mythology of the Aztec Empire includes the twins of Quetzalcoatl (meaning “Feathered Serpent” in Nahuatl, a language used by the Aztec People of Central Mexico) and Tezcatlipoca (“Smoking Mirror”), in addition to the siblings of Huitzilopochtli (“Hummingbird on the Left”) and Coyolxauhqui (“Face Painted with Bells”). These mythic examples found in the archaeological record shape the pan-Mesoamerican worldview, centered on contrasting duality and paradoxical pairings

Twins Among the Low Mass Spectroscopic Binaries

We report an analysis of twins of spectral types F or later in the 9th Catalog of Spectroscopic Binaries (SB9). Twins, the components of binaries with mass ratio within 2% of 1.0, are found among the binaries with primaries of F and G spectral type. They are most prominent among the binaries with periods less than 43 days, a cutoff first identified by Lucy. Within the subsample of binaries with P<43 days, the twins do not differ from the other binaries in their distributions of periods (median P~7d), masses, or orbital eccentricities. Combining the mass ratio distribution in the SB9 in the mass range 0.6 to 0.85 Msun with that measured by Mazeh et al. for binaries in the Carney-Latham high proper motion survey, we estimate that the frequency of twins in a large sample of spectroscopic binaries is about 3%. Current theoretical understanding indicates that accretion of high specific angular momentum material by a protobinary tends to equalize its masses. We speculate that the excess of twins is produced in those star forming regions where the accretion processes were able to proceed to completion for a minority of protobinaries. This predicts that the components of a young twin may appear to differ in age and that, in a sample of spectroscopic binaries in a star formation region, the twins are, on average, older than the binaries with mass ratios much smaller than 1.Comment: Accepted by the Astronomical Journa