The equal environments assumption of classical twin studies may not hold

The classical twin method – comprising comparisons of monozygotic (MZ) and dizygotic (DZ) twins – in the domain of cognitive abilities and attainments has led to wide acceptance of results suggesting a large amount of additive genetic variance, with far-reaching implications both for the nature of future studies on the causes of cognitive variance and for intervention policies, as in education. However, this interpretation is only valid if the method observes a number of conditions, which have to hold. Here, we show that the most crucial of these, namely, the equal environments assumption (EEA), may not hold. Consequently, differences in twin correlations might be at least partly explained by treatment effects from parents, teachers, peers, and so on. In addition, well-known interactions at various levels confound the model of simple additive effects on which the classical twin method is predicated and results are interpreted. For example, at a socio-cognitive level, DZ twins may respond to treatments differently from MZ twins. This interaction may further explain MZ–DZ correlation differences. There is abundant evidence for such interactive effects in published twin data. We suggest that there is a need for a more thorough examination of these problems

The equal environments assumption of classical twin studies may not hold

The classical twin method – comprising comparisons of monozygotic (MZ) and dizygotic (DZ) twins – in the domain of cognitive abilities and attainments has led to wide acceptance of results suggesting a large amount of additive genetic variance, with far-reaching implications both for the nature of future studies on the causes of cognitive variance and for intervention policies, as in education. However, this interpretation is only valid if the method observes a number of conditions, which have to hold. Here, we show that the most crucial of these, namely, the equal environments assumption (EEA), may not hold. Consequently, differences in twin correlations might be at least partly explained by treatment effects from parents, teachers, peers, and so on. In addition, well-known interactions at various levels confound the model of simple additive effects on which the classical twin method is predicated and results are interpreted. For example, at a socio-cognitive level, DZ twins may respond to treatments differently from MZ twins. This interaction may further explain MZ–DZ correlation differences. There is abundant evidence for such interactive effects in published twin data. We suggest that there is a need for a more thorough examination of these problems

High-latitude HF-induced airglow displaced equatorwards of the pump beam

HF-induced airglow at 630 nm was observed by the Digital All-sky Imager, located near Skibotn in Norway, at F-region altitudes above the EISCAT HF facility near Tromsø on 21 February 1999. The transmitter was operated in a 4-min on, 4-min off sequence at 4.04 MHz O-mode with the beam pointing vertically. The airglow reached a peak intensity of about 100 R above background and appeared equatorward of the HF beam’s projection on the reflection altitude, which was obtained from ionograms. Generally, the region of maximum airglow was displaced towards the magnetic field line (zenith angle = 12.8° S) passing through the HF facility. This is a unique feature of these observations. From mid-latitude studies, such airglow is thought to be excited either by electrons energised to several eV by plasma turbulence, or by thermal electron temperature enhancement. Such localisation towards the magnetic field is unexpected for both mechanisms of airglow generation and suggests this feature may be important at high latitudes