Investigators have used twin methodology to determine how much of the family aggregation described is due to genetic factors and how much to the environment. Twin studies in adults have shown consistently higher concordance rates in monozygotic twins compared to dizygotic twins for CFS/ME, with the monozygotic correlation usually at least twice that of the dizygotic correlation11,12,13,14
(for example 0.55 and 0.1914
and 0.43 and 0.16,12
respectively). As environmental factors (eg, infections or parental up‐bringing) during early life should have been similar within monozygotic and dizygotic twin pairs, the difference between monozygotic twins who share 100% of their DNA and dizygotic twins, who on average share 50%, can be used to calculate the contribution made by genetic heritability. The difference in concordance rates increases with increasingly stringent case definition in adults.14
This provides evidence that there is a core CFS/ME group among whom genetic factors play an important role.
Repeating the studies in children is difficult for many reasons, not least because there are currently no agreed criteria for CFS/ME in children. The Royal College of Paediatrics and Child Health do not recommend a minimum time frame before making a diagnosis in children but merely require that they have fatigue which is disabling and without another identifiable cause.6
The NICE guidance for CFS/ME published in August 2007 suggest that a diagnosis of CFS/ME is made in children when symptoms have persisted for 3 months.15
Nevertheless, twin studies investigating fatigue in children are consistent with those in adults. In the UK, carers of 670 twin pairs were questioned about disabling fatigue lasting more than a week and more than a month. In both cases the concordance was higher in monozygotic twins compared to dizygotic twins (0.81 vs 0.59 for 1 week, 0.75 vs 0.47 for 1 month).16
This suggests that the genetic contribution to the experience of disabling fatigue is high. This study was extended and length of time for fatigue was defined as a few days, more than a week and more than 1, 3 or 6 months, and the relationship between fatigue and depression was investigated in 1468 pairs of twins.17
For short duration fatigue, genetic heritability was high and no shared environmental contribution (from environmental factors shared by twins, as many background family‐based factors would be expected to be) could be detected. However, for prolonged fatigue, there appeared to be substantial environmental influences and a modest genetic contribution.