A second effect is "G × E correlation", in which certain alleles tend to accompany certain environments. If a gene causes a parent to enjoy reading, then children inheriting this allele are likely to be raised in households with books due to GE correlation: one or both of their parents has the allele and therefore will accumulate a book collection "and" pass on the book-reading allele. Such effects can be tested by measuring the purported environmental correlate (in this case books in the home) directly.