To better understand what causes what and also to determine what role genetics play, researchers in this study looked at reading achievement and independent reading in 436 pairs of identical and same-sex nonidentical twins at age 10 and again a year later at 11. Reading achievement was assessed using standard measures of word recognition (recognizing single words) and reading comprehension. Independent reading was assessed by asking each twin questions about his or her motivation to read. Parents estimated how often their children read for pleasure.From a paper by:
The study found that children's reading achievement at age 10 predicted their independent reading at 11, regardless of how much independent reading they were doing at 10, suggesting that reading achievement influenced later independent reading. The reverse was not true. After accounting for reading achievement at age 10, independent reading at 10 didn't predict reading achievement at 11.
Nicole Harlaar, Kirby Deater-Deckard, Lee A. Thompson, Laura S. DeThorne, Stephen A. Petrill. Associations Between Reading Achievement and Independent Reading in Early Elementary School: A Genetically Informative Cross-Lagged Study. Child Development, 2011