Interesting discussion, and it's good to see so many points of view put forward without rancour. But I still can't decide on my original query. There has been partial reporting- (over?) dramatisation against self- exculpation perhaps? The circumstances are not as clear cut as the BBC report suggested. So it might be better to recast it as a hypothetical rather than a specific case.

(1) Research on twins is a valuable method of determining the relative strength of genetic and cultural effects on development. This is useful because it can assist with educational and other social strategies. It's also dangerous because it can be used to justify preconceived social assumptions.

(2) Twins are a scarce resource, and still scarcer are identical twins who have been subject to different developmental experiences. Identical twins also have an unusual relationship to each other, in that they share exactly the same genetic basis. The part of their mind that is dependent on its hardware structure can be assumed to be much closer than other pairs of humans.

(3) Children have a right to the best upbringing that we can provide for them. This includes the physical situation, and also the non- physical environment ("spiritual" is the term often used, but it's best avoided because of its other religious connotations). There may well be a tradeoff between the physical resources available- parents can provide less for two than one- and the benefits (or as one psychologist is said to have claimed above, problems) of having another human being to whom you are uniquely close.

(4) To gain the benefits of studies of twins, they have to be studied, and to have statistical significance, the studies must include as large a nummber of subjects as possible. As this may well include intimate details of identifiabvle families, the privacy of those families must not be invaded unnecessarily. On the other hand, the individual subjects have the right to know what has been done to them and why. This could include an explanation, say, that the disclosure of the fact that they are a twin could render the results invalid (and a demonstration of that).

Do researchers make clear in their justification for a study the exact tradeoffs they have made in these dilemmas, and is there some publicly debated ethical standard by which it can be agreed that it is worthwhile?