When I’m asked to pinpoint the moment my belief in ghosts started to waver, I choose the time I was mistaken for one in Gettysburg, Pennsylvania, when I was 27. Until then, I’d enjoyed several years of ghost-hunting, and was convinced I’d encountered many supernatural entities. Growing up in the 80s, my interest in the paranormal was piqued by TV shows such as In Search of … and Unsolved Mysteries, which documented cases of unexplained phenomena. In my 20s, I got hooked by the possibility of experiencing the “other side” and the idea that life continued after death. While browsing ghost stories on the internet, I found a local ghost-hunting group in Philadelphia and joined them on trips to cemeteries and allegedly haunted buildings such as Fort Mifflin and the Eastern State Penitentiary. There were six of us around the same age, all determined to prove ghosts were real. To us, anomalies in our photos such as glowing orbs or mist signalled spirits, as did any surprising noise caught on cassette. Like most ghost hunters, we operated in the dark, which inevitably fed our imaginations. We’d turn up with cases of equipment we’d seen ghost-hunters use on TV, such as electromagnetic field and motion detectors. We had no real understanding of what they did, but a beep or a flashing light had to mean something. Museum staff, owners of old mansions and worried residents who’d heard strange noises in their apartments would all be told the same thing: “Yes, this place is haunted.” Some people reacted with fear, others with relief – “I knew I wasn’t crazy!” A paranormal conference in 1999 led to that fateful Gettysburg encounter. We were at the US civil war battlefield, and one night went out to an area we’d heard was particularly haunted (a ranger had told us even he and his colleagues wouldn’t set foot there). We were setting up our cameras and tape recorders in some woods when three cars appeared. A group spilled out waving laser pointers. Annoyed, I marched towards them, shouting and waving my arms. The newcomers stopped dead, backed off and finally drove away. Next morning, rumours were rife that a group had sighted an apparition. Intrigued, I learned their “ghost” had emerged from the woodland, screaming unintelligibly in “a hollow voice”. “Are you kidding?” I said. “That was me!” I described the event from my perspective, but they wouldn’t listen. Finally, one of them yelled, “Stop trying to steal our spotlight!” That’s when it clicked: how many times had I avoided the obvious explanation, just because I was enjoying the excitement? It was horrible to realise I’d spread misinformation. I started looking into the work of people like Joe Nickell, a sceptical, science-based paranormal investigator. I read up on photography and learned that “orbs” were caused by camera flash reflecting off microscopic dust particles, “ectoplasmic mist” was my own breath rising in front of the lens and transparent apparitions were the result of long exposures. Devices used to “detect energy” responded to any electromagnetic field; those used to communicate with spirits could be set off by two-way radios. Two decades on, I took over from Nickell as chief investigator for the Committee for Skeptical Inquiry, which was set up in the 1970s to challenge the growth in claims of ghost and monster sightings, and the efficacy of psychics, mediums and practitioners of pseudo-scientific alternative medicine. I’ll pick an unsolved mystery, comb through primary sources like police reports dating as far back as the 1800s, chase first-hand accounts and visit the sites of historic “hauntings”, re-staging photographs using vintage equipment. I feel genuine remorse for the times I confirmed the suspicions of people who were worried their house was haunted, leaving them scared while I chased the next thrill. Now I gain much greater fulfilment from teaching people how to use critical thinking to interpret the situation themselves. I like to think I’m like Velma from Scooby Doo, the brains of Mystery Incorporated: I’d love to find a ghost I couldn’t explain away, or an alien or Bigfoot. But they’re going to have to work pretty damn hard to convince me. As told to Chris Broughton