There is a widespread belief that enthusiastic recreational reading must be related to good life outcomes. This belief takes many forms but is perhaps most common in the almost cargo cult like belief in the osmotic acquisition of personal traits through recreational reading. Specifically, many people believe that if you read a lot of literary fiction (narratives with a primary focus on interpersonal relationships) you will become more empathetic, that if you read books with great representational diversity, you will become more tolerant. There are numerous parallel beliefs such as that mysteries foster critical thinking. The belief is that recreational reading fosters all these positive attributes which in turn foster good life outcomes.
The evidence to support any of these assumptions is exceptionally thin and there is a lot of counterfactual evidence. However, there is a good evidentiary basis for believing that purposeful reading (as opposed to recreational reading) is indeed related in a causal fashion to good life outcomes.
But why then the widespread belief in the beneficial influences of recreational reading and why does that not show up in the data?
My speculation is that recreational reading per se has no impact on good life outcomes. It is just another recreational activity. However, when recreational reading exposes someone to new ideas or interests, that is likely to lead to purposeful reading in those new areas. In other words, it is not the recreational reading that has the causal affect. Instead recreational reading potentially serves as a catalyst to other interests which might in turn generate purposeful reading (where the evidence is strong for positive causal correlates).
I just came across an example of this in reading Charles Darwin's Autobiography. Darwin has been thinking, studying and reading intensively around the issue of evolution.
After my return to England it appeared to me that by following the example of Lyell in Geology, and by collecting all facts which bore in any way on the variation of animals and plants under domestication and nature, some light might perhaps be thrown on the whole subject. My first note-book was opened in July 1837. I worked on true Baconian principles, and without any theory collected facts on a wholesale scale, more especially with respect to domesticated productions, by printed enquiries, by conversation with skilful breeders and gardeners, and by extensive reading. When I see the list of books of all kinds which I read and abstracted, including whole series of Journals and Transactions, I am surprised at my industry. I soon perceived that selection was the keystone of man's success in making useful races of animals and plants. But how selection could be applied to organisms living in a state of nature remained for some time a mystery to me.In the middle of this intense and purposeful effort, Darwin indulges in some recreational reading.
In October 1838, that is, fifteen months after I had begun my systematic enquiry, I happened to read for amusement 'Malthus on Population,' and being well prepared to appreciate the struggle for existence which everywhere goes on from long-continued observation of the habits of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved, and unfavourable ones to be destroyed. The result of this would be the formation of new species. Here then I had at last got a theory by which to work; but I was so anxious to avoid prejudice, that I determined not for some time to write even the briefest sketch of it. In June 1842 I first allowed myself the satisfaction of writing a very brief abstract of my theory in pencil in 35 pages; and this was enlarged during the summer of 1844 into one of 230 pages, which I had fairly copied out and still possess.Here then you have the ground laid through intense and purposeful reading and cogitation. Recreational reading provides the spark leading to the new insight. This type of event is what I suspect is the genesis for the widespread belief that recreational reading has real world positive consequences. It does, but only in the context of preparation and purposefulness.
And that predicate requirement is why, I suspect, that there is such little data supporting the causative relationship between recreational reading and positive life outcomes. Most recreational reading occurs in the absence of the preparation and purposefulness in evidence with Darwin. Absent that context of preparation and purposefulness, the great bulk of recreational reading has no real world consequence.
The upshot of this line of thinking is this.
Purposeful reading has real world positive consequences which fuels the assumption that all reading has real world positive outcomes.
Recreational reading which is a material, if not dominant, portion of all reading, has no measurable causative relationship with positive real world outcomes.
In a small portion of cases, those who read purposefully also read recreationally. Under these circumstances, it occasionally happens that the recreational reading catalyzes the purposeful reading in some beneficial fashion. This further fuels the impression that recreational reading is linked to positive outcomes when in fact that only occurs through the mediation of purposeful reading.
UPDATE: It's not just reading that prompts new ideas of course. Here is a structurally similar set of facts with entirely different inputs. From 'From Atoms to Bits': A Brilliant Visual History of American Ideas by Derek Thompson.
One night in the the spring of 1983, the scientist Kary Mullis was driving with his girlfriend along Highway 128 from Berkeley to Mendocino, California. As Mullis took in the perfume of California buckeyes swinging their blossoms along the road, his mind wandered back to his job as a chemist. He was thinking about human DNA. Specifically, he was thinking about how to replicate human DNA. And it was there, at "mile marker 46.7 on Highway 128,” as he specified a decade later in his Nobel Lecture, that he experienced that rare and often apocryphal moment of invention—a eureka.