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I'm looking for a short story about the first human trip to Mars.

In it, the further the astronauts go from Earth the smarter they get. At the halfway point they have super-intelligence and realize that there are entities on Earth that suppress our intelligence. They then start losing their intelligence as the Mars based entities begin to exert their influence on them.

Unwelcome Tenant by Roger Dee

Astronaut Mayard reaches the balance point between Earth's gravity and Mars' gravity. At which point he becomes aware of an immaterial mental parasite in his brain, which exits and flies way screaming. Mayard suddenly becomes super intelligent. He realizes that everybody on Earth has the parasites.

But it all turns to worms when he travels a bit further. Turns out that Mars is loaded with the parasites as well, and one immediately takes possession of him.

http://www.isfdb.org/cgi-bin/title.cgi?58643

Available in the Internet Archive

https://archive.org/details/Planet_Stories_v04n07_1950-Summer/page/n97

Author Colin Wilson was inspired by the story to add a similar scene in his novel The Mind Parasites. He does mention this in the novel's introduction, but does not mention the story name.