The Alfa Effect was the term used to describe a unique transporter accident that occurred in the year 2266 when the Federation starship USS Enterprise visited the planet Alfa 177. Rare elements on the planet caused Captain James T. Kirk to be divided into two different people when he was transported up from the planet, with one Kirk representing all of his positive qualities and the other all of his negative ones. A later attempt to transport a native animal up from the surface resulted in a similar split, with one version being timid and the other savage. When it was determined that neither Kirk could exist as individual entities for long, the crew attempted to test a recombination process on the two animals, which apparently died from shock. However, Doctor Leonard McCoy deduced that the death only occurred because the animal couldn't understand what was happening, but theorised that the rational Kirk would be able to understand what was being done to him and accept it. Faced with death or a chance at survival, the two Kirks were eventually recombined into one entity.
Over the following decades, a great deal of research was done on the Alfa Effect and several papers written about it, although Kirk never read any of them himself, finding the experience too disturbing to recall.
In 2288, Colonel Demme Gast, a rogue Pavakian objecting to her people's planned peace treaty with their long-time enemies, was able to use the various research done on the effect over the years to duplicate the Alfa Effect, believing that her research had allowed her to limit the personality polarisation that afflicted the two Kirks. As a result, one Gast was able to commit various murders in an attempt to sabotage the peace talks while the other appeared in public to present a seemingly perfect alibi. The deception was eventually exposed, but by that point the Gasts had been apart for too long and died when the Enterprise crew attempted to put them back together. Despite Gast's confidence that she had minimised the personality polarisation, towards the end the two Gasts had very different reactions to their fate, with one accepting their failure as the other raged against it. (TOS novel: Foul Deeds Will Rise)