Twin children in volunteer registries: biases in parental participation and reporting

The Mercurio Project and Twins. How to use a registry

An essential element in any investigation of environment-genetics interaction is twin registries. At present there exist two types, one based on the whole population and another relying on voluntary participation. The second type is an important source of epidemiological information regarding the presence or absence of a disease (concordance) between members of monozygotic and dizygotic pairs. However, this type presents two serious drawbacks: the data do not cover all twins, and since the twins enrolled are those who require the greatest assistance, this results in a serious selection bias [2]. On the contrary, the first type has neither of these defects and represents an invaluable source of information for epidemiological studies on the concordance of pysiological features, morbidity and mortality, together with genetic and environmental influences and their interaction [1, 3-5].

April 1993 saw the first enrolment in a Multiple Pregnancies Registry designed to collect data from 47 Italian hospitals. This registry forms part of a larger survey, the “Mercurio Project”, originally set up in April 1992 under the aegis of the Italian Multicentre Birth Defects Registry (IPIMC). The aims of the registry are the following:

(a) to set up longitudinal epidemiological studies on the health, morbidity and mortality of twins

(b) to assess the role of environmental-genetic interaction in determining childhood behaviour and disease (using case-control studies where each twin is matched with his or her cotwin)

(c) to draw the attention of socio-sanitary personnel to twins and their special needs.

The epidemiology of genetic epidemiology

Familial aggregation for disease is important; strong familial risk factors must exist even if the increased risk to a relative of an affected individual is modest. It is in practice difficult, however, to conduct studies in genetic epidemiology which conform to strict epidemiological principles. For twin studies there are two major questions: Are twins 'no different' from the population on which inference is to be made? Are study twins 'no different' to twins in the population? The importance of each question of bias depends on the scientific question, the trait(s) studied, and sampling issues. The strength of the twin design is its ability to refute the null hypothesis that genetic factors do not explain variation in a trait. Following the Popperian paradigm, alternate hypotheses should be considered in depth (both theoretically and empirically), with a design and sample size sufficient to exclude not just naive explanations. More sophisticated statistical techniques are now being applied, so the philosophy, assumptions, and limitations of statistical modelling must be appreciated. The concept of 'heritability' has, in the past, been misunderstood and misused. New advances in DNA technology promise to revolutionise epidemiological thinking, and so case-control-pedigree designs may well become standard tools. The strengths and limitations of studies based on related individuals as the sampling unit are discussed.

IQ similarity in monozygotic and dizygotic twin children: effects of the same versus different examiners: a research note

The Wechsler Intelligence Scale for Children--Revised was administered to two independently identified samples of monozygotic (MZ) and dizygotic (DZ) twin children. Co-twins in the first sample were tested by the same examiner, while co-twins in the second sample were tested by different examiners. IQ intraclass correlations were similar for the two MZ twin samples (.84 and .88), and identical for the two DZ twin samples (.54). The implications of this analysis are discussed with reference to twin study methodology, research on IQ similarity and replication of child development research.