In a nutshell, evolution describes the fact that in a given set of organisms, those which manage to survive are the same ones that do most of the breeding. It naturally follows that the traits of the survivors become the most prevalent in the species. Perhaps one of the first traits to be promoted by evolution was the survival instinct itself, since the two reinforce one another so elegantly.
As part of the survival instinct, most animals (including humans) react to sudden, unexpected stimuli with a startle reaction, which includes reflexive movement away from the stimulus and a contraction of the muscles in the limbs. It also causes changes in blood pressure, respiration, and breathing. In a normal individual, the muscular reaction subsides within a couple seconds if no real threat is detected, but for a sufferer of the Jumping Frenchman of Maine Disorder, an unexpected stimulus results in a somewhat different experience.
An individual with this disorder has a genetic mutation that prevents "exciting" signals in the nervous system from being regulated, which causes a number of bizarre irregularities in their startle response. Most notably, an event which might startle a normal person will result in an extended, grossly exaggerated response from a "jumper," including crying out, flailing limbs, twitching, and sometimes convulsions. Because a jumper is almost immediately susceptible to another jump soon after an episode ends, there have been reports that sufferers are sometimes teased mercilessly by people who find the reaction amusing, and trigger it repeatedly.
Another curious abnormality caused by this disorder is a sufferer's automatic reflex to obey any order that is delivered suddenly. For example, if one uses a sharp, quick voice to order a jumper to throw the object in their hands, they will throw it without hesitation; if they are similarly told to strike a person, they will strike that person, even if it is a loved one. Very often, if an individual with this disorder hears a phrase that is unfamiliar or spoken in a loud voice, they will uncontrollably repeat that phrase back, a phenomenon known as echolalia.
The Jumping Frenchman of Maine Disorder was first described by G. M. Beard in 1878 after observing the effect in French-Canadian lumberjacks in the Moosehead Lake area of Maine:
“One of the jumpers while sitting in his chair with a knife in his hand was told to throw it, and he threw it quickly, so that it struck in a beam opposite; at the same time he repeated the order to throw it…. When the commands are uttered in a quick loud voice the jumper repeats the order. When told to strike, he strikes, when told to throw it, he throws it, whatever he has in his hands…. all of these phenomena were indeed but parts of the general condition known as, jumping. It was not necessary that the sound should come from a human being: any sudden or unexpected noise, as the explosion of a gun or pistol, the falling of a window, or the slamming of the door, provided it be unexpected and loud enough, would cause the jumpers to exhibit some one or all of these phenomena….” (Beard, 1880a, pp. 487-490)
This highly unusual disorder has also been observed in a few other parts of the world, including Siberia and Malaysia. Jumping Frenchman is a form of Hyperexplexia, a more generic term descriptive of any exaggerated startle response. Jumping Frenchman bears similarities to Stiff Baby Syndrome, a condition where a person has an exaggerated startle response from the time they are born, and a tendency as a young child to occasionally stiffen their whole body in response to surprise stimuli. This causes the child to fall to the floor like a log when surprised, not unlike fainting goats. Jumping Frenchman and Stiff Baby are so similar that they are thought by many to be the same disorder. When Beard first described Jumping Frenchman in 1878, Dr. Gilles de la Tourette attempted to have it classified as part of the syndrome which bore his name, but this notion was rejected due to the distinct differences between Tourette Syndrome and Jumping Frenchman.
Most evidence seems to indicate that hyperexplexia is caused by a genetic mutation which prevents neurons from receiving the amino acid glycine. Glycine in an inhibitory neurotransmitter common in the spinal cord, and it prevents the central nervous system from overreacting to stimuli. The poison strychnine also inhibits glycine reception, and the symptoms it causes are very similar to hyperexplexia.
In the case of the original test subjects in Maine, it appears that the problem mutation might have been caused by inbreeding, however some researchers believe that the disorder is psychological rather than neurological. The debate is still open. Either way, it is always fascinating to see what happens when the human brain experiences a short circuit. Indeed, truth is sometimes stranger than fiction... sometimes to a startling degree.