Sometimes when watching period shows (set in USA, Canada, or England), I see a lot of reference to spiritualism (especially seances). I know about Doyle and Houdini and how they viewed it. But is it over-represented in fiction today?
Oddly, despite the time-frame indicated by Wikipedia, I usually associated with the very early 20th century instead of the 19th. Probably because of the shows I watch.
But how popular was it, and how seriously was it taken? Did most of the people attending seances really believe in them? Did it generally co-exist with or seek to replace conventional religion?
Boiling a very complicated historical situation down to a few lines; spiritualism has existed as a practice throughout history, but for practical purposes we can date the 19th/20th century Spiritualism movement to the activity of the Fox sisters circa 1850.
Seances in which spirits were said to communicate with the living continued as a sort of occult subculture thereafter, waxing and waning over time, until the events of the First World War and the Spanish Flu epidemic (roughly 1914-1919). Many millions of people died untimely deaths due to war and disease during that period, leading to a comparatively massive renaissance of interest in Spiritualism during the 1920s and '30s, as grieving relatives hoped to be able to communicate with their lost loved ones.
Writing in 1932, Julien Proskauer - who was, like Houdini, a magician dedicated to exposing fraudulent mediums - estimated the number of people "suckered" by hoax seances, numerological and fortune telling sessions, etc. at 30,000,000 per year, and their losses at $125,000,000. In any case, yes, it was a major cultural phenomenon and business, very widely reported upon in the media, significantly influencing the pop-culture of its day, etc.
It's difficult to estimate how seriously it was taken with any accuracy, but there certainly were many "true believers" as well as large numbers of both spiritualistic con-artists and skeptics.
Pre-WW1 the trend was to co-exist with conventional religion but, fueled by the existential horror of the war and epidemic, there was a movement afterward towards Spiritualism supplanting conventional religion. In a filmed interview shortly before his death, Arthur Conan Doyle - who was clearly utterly sincere in his belief - noted that he expected that to happen in the future. It didn't, of course, but Spiritualist churches still exist today, as do "New Age" trance channelers and so-on. The big difference is that very few modern practitioners attempt the sort of "physical manifestations" (ectoplasm, table-tipping, etc.) that were convincingly debunked as magic tricks by generations of skeptics during the early/mid 20th century.
IMO Spiritualism is frequently represented in modern fiction primarily because it's a naturally dramatic subject, but given the history, I wouldn't describe it as an over-representation.