To investigate the fundamental causes of bloat, six arti cial random binary tree search spaces are presented. Fitness is given by program syntax (the genetic programming genotype). GP populations are evolved on both random problems and problems with \building blocks". These are compared to problems with explicit ine ective code (introns, junk code, inviable code). Our results suggest the entropy random walk explanation of bloat remains viable. The hard building block problem might be used in further studies, e.g. of standard subtree crossover.