Grabiya's Theorem was the posited solution put forth by Terran gene-wrights during the Great Crusade era of the late 30th Millennium to solve the diminishing stability of the Space Marines' gene-seed stores through over-use, due to the rapid expansion of the nascent Imperium of Man by the Emperor of Mankind's Space Marine Legions in this era. This theorem demonstrated that a Primarch's genetic code could be used to stabilise and expand existing Astartes gene-seed stocks with what was hoped to be "minimal deviation" and mutation from the baseline genetic code.
As the Great Crusade continued the expansion of the nascent Imperium across the galaxy, the discovery of the Primarchs and their newly adopted homeworlds helped to stem an impending crisis that was not widely known of at the time outside of the exalted ranks of the Imperium's ruling War Council: namely, the diminishing stability of the Astartes' gene-seed itself through over-use and the increasing need of the Imperium for ever greater numbers of Space Marines in the field. This was a matter that was only exacerbated as the Expeditionary Fleets of the Great Crusade pushed ever wider afield into the galaxy. Imperial forces could no longer be concentrated as easily as before, and attrition was taking its toll as the years of near-constant battle became decades. Slowing the pace of the Great Crusade's progress was simply not an option for the Emperor, and so the simple truth was that more Space Marines were needed and they needed to be created faster than had been done using the gene-seed cultivation and Astartes organ implantation methods of the past.
A secret conclave of gene-wrights under the Emperor's direct supervision posited the solution that became known as Grabiya's Theorem, which demonstrated that a Primarch's genetic code could be used to stabilise and expand Astartes gene-seed stocks with what was hoped to be "minimal deviation." Alongside this accelerated gene-culturing technique, other previously unavailable genetic engineering technologies were put into effect, reducing the processing time required to create a battle-worthy Space Marine to a single Terran year in some cases. Such accelerated gene-seed cultivation techniques, along with absent, inadequate or over-forceful psycho-doctrination techniques, were later found to have produced unseen fundamental flaws in the Astartes subjected to them. Many Imperial savants since have come to believe that the drive to create larger Space Marine Legions at an accelerated speed played a crucial role in the degradation of the sanity and psychological make-up of certain Space Marine Legions and paved the way for the horror of the Horus Heresy that was to come in the early 31st Millennium.
- The Horus Heresy: Betrayal, Book One by Alan Bligh, pp. 29-30
- White Dwarf 98 (UK), "Chapter Approved: The Origin of the Legiones Astartes," by Rick Priestley, pg. 16