Twins are the gold dust of numerous studies of the interaction between genetic and environmental factors in child development.

In the UK, their value is regularly reinforced by findings from TEDS [1], the Twins Early Development Study, which is keeping track of all pairs born in England and Wales between 1994 and 1996.

TEDS cohort data are helping to prise open the relationship between the environment and the genetic components of hyperactivity and academic achievement.

Information from the same source has shed light on the connection between early language problems and later reading difficulty.

Similarly, based on their classroom experience, it has been possible to study the mental health outcomes of separated twins, and consequently to improve understanding of the emotional needs of all children.

As Angelica Ronald [2], a colleague of TEDS project director Robert Plomin at the London Institute of Psychiatry, explains:

“The beauty of twin studies is that they provide psychologists with a natural experimental design – there’s no need for any additional control group.

“This natural design comes about because there are two types of twins: those who share all their genes (because they were formed from the same egg which split early on in development), called identical or monozygotic twins, and those who, just like non-twin siblings, share on average half their genes (they are formed from two separate eggs), called fraternal or dizygotic twins.”

So twin designs give behavior geneticists the wherewithal to pick apart the tightly knotted threads of nature and nurture.

Angelica Ronald’s specialism is autism. "If genes influence variation in autistic behaviors, identical twin pairs who share all their genes will be highly similar in their degree of autistic behaviors whereas fraternal twins will be much less similar. This is what we have found," she says.

All this being so and the educational development of twins being so central to so much fruitful research, there is something paradoxical about a UK report and campaign to remedy claims of widespread misunderstanding in the UK education system of the basic needs of multiple birth children.

The Twins and Multiple Birth Association (TAMBA [3]) is pressing the schools adjudicator to alter the school admissions code to ensure that it is no longer the case that hundreds of pairs a year are allocated places in different schools.

The TAMBA report suggests two other failings in the system: it criticizes the policy of splitting twins up by putting them in different classes without taking parents' wishes into account, and it condemns the inflexibility it claims many local authorities show in refusing to allow parents of premature multiples the chance to hold them back a school year.

TAMBA made its own survey of around 1,000 parents asking them about their children’s needs from pre-school through primary to secondary and about how well they were being met.

Ironically, their report also draws on the TEDS evidence that twins separated at the start of primary school have more emotional problems on average (shyness, withdrawal, depression, anxiety) than non-separated twins.

Tamba's chief executive, Keith Reed, wants the adjudicator to rule that if one twin was given the thirtieth place in a class, an exception could be made to the 30-pupil rule to allow the other twin into the class as well.

He is quoted by The Guardian [4] as having been taken aback when the report uncovered how widespread the problem it was. “What we found was that at least 20% of parents were not even consulted about whether their children were split up or kept together – they were simply told the school had a policy, and they would have to fit in with that.”

"There seems to be a well-meaning assumption that you help twins by splitting them up and helping them to do things independently, but the fact is that's not always the case. In far too many places, parents' views simply aren't being taken into account."

• For Prevention Action coverage of TEDS cohort studies, see A pair of genes to suit every occasion? [5] and Toward a new reading of language development [6].