Extending the classical twin method, Arthur R. Jensen claims to find unique estimates of the variance and covariance of the genetic and environmental components of IQ. The analyses presented in this paper show that a wide variety of parameter estimates are compatible with his model and data. But the extent of indeterminancy is deemed to be even wider. Furthermore, his model is shown to be not immutable. In particular, Jensen's assumption that one twin's environment is as highly correlated with the other twin's genotype as with his own genotype is questioned. It is stated that the twin method cannot be used to extract meaningful estimates of the variances, and covariance, of the genetic and environmental components of human intelligence. Finally, it is said that any plausible model for the resemblance between the twins will have so many more unknown parameters than observations that the task is futile. (Author/AM)