Inscribed in pencil at the top left corner of one of the world's most famous and expensive paintings are the words: "Could only have been painted by a madman!"
It is believed Munch pencilled in the sentence after being hurt by criticism about his mental state when his painting was exhibited in 1895.
The miniscule inscription was first mentioned in 1904, 11 years after when Munch painted The Scream in 1893. A Danish art critic, who saw the painting at an exhibition in Copenhagen, thought it was written by a member of the public.
But research by the National Museum of Norway reveals the handwriting belonged to Munch.
The painting was photographed using an infrared camera, making the small writing stand out from the painted background so it can be studied closely. The handwriting was then compared to Munch's notes and letters.
Thierry Ford, paintings conservator at the National Museum, said: "The writing has always been visible to the naked eye, but it's been very difficult to interpret. Through a microscope, you can see that the pencil lines are physically on top of the paint and have been applied after the painting was finished.
"We chose to photograph it with an infrared camera to get a clearer picture of the inscription. In an infrared photo, the carbon from the pencil stands out more clearly and makes handwriting analysis easier. And you don't have to impact the painting itself."
It is believed that the inscription was written as the artist was affected by questions about his mental state when The Scream was exhibited for the first time to a Norwegian audience in October 1895.
Henrik Grosch, then director of the Norwegian Museum of Decorative Arts and Design, wrote the paintings showed that one can no longer "consider Munch a serious man with a normal brain".
In a discussion about Munch's art held by the Student Society in Kristiania, medical student Johan Scharffenberg said the painting Self-Portrait with Cigarette indicated Munch was not a normal human being.
The criticism affected Munch, who was known to have had mental health problems throughout his life. As late as the 1930s, Munch brought up this episode several times and his notes are full of self-defence.
Mai Britt Guleng, curator at the National Museum, said: "The theory is that Munch wrote this after hearing Scharffenberg's judgment on his mental health, sometime in or after 1895. It is reasonable to assume that he did it quite soon after, either during or following the exhibition in Kristiania.
"Munch was also generally concerned about the idea of hereditary disease in the family. Both his father and grandfather suffered from what was then referred to as melancholy and his sister Laura Munch had been admitted to Gaustad Psychiatric Hospital."
She added: "The inscription can be read as an ironic comment, but at the same time as an expression of the artist's vulnerability. Writing on the finished painting shows that creating for Munch was a continuous process."
Infrared photos are commonly used to detect anything hidden behind a painting. But in the case of The Scream, it did not reveal any unknown sketches or overpaintings.
On the back of the original painting, there is a partial version of a slightly different composition. Munch rejected this, turned the canvas over and painted The Scream.
The Norwegian artist's work has been hailed as "one of the most iconic images in art history" by Hartwig Fischer, director of the British Museum.
It has become a well-known expression of human anxiety, becoming the face of the 90s Hollywood Scream horror film series and influencing the modern-day scream emoji.
It is one of the world's most expensive paintings ever sold - fetching $120 million (£74 million) at auction at Sotheby's in New York in 2012. At the time of the sale, it was the most expensive painting ever sold at an auction.
Munch's Self-Portrait with Cigarette and The Scream will be displayed at the National Museum of Norway from 2022.