Peter H. Schonemann
Environmental versus genetic variance component models for identical twins:
A critique of Jinks and Fulker's reanalysis of the Shields data
Cahiers de Psychologie Cognitive / European Bulletin of Cognitive Psychology, 1990, 10, 451-473.
It is shown that the genetic model Jinks and Fulker (1970) fitted to the Shields' (1962) twin data is qualitatively inconsistent with systematic trends in these data and, as a result, produces an inordinately large proportion of negative variance estimates.
In contrast, a purely environmental model yields qualitative predictions consistent with the Shields data and admissible parameter estimates throughout. Quantitatively, it fits the Shields data twice as well as Jinks and Fulker's genetic model. Hence their far-reaching conclusions are not supported by the Shields data.
This reevaluation illustrates that purely descriptive models, even if they were used with circumspection, remain intrinsically inconclusive about nature/nurture questions because the possibility can never be ruled out that other models may fit the same data even better.