In November 1990, Shawcross went on trial for the ten murders that had occurred in Monroe County, which included all victims except for Elizabeth Gibson, who had been killed in neighbouring Wayne County. The trial was a national media event, extensively televised and widely viewed.
Shawcross’ defence team tried to build a case based on an insanity plea, citing various mitigating factors, such as his upbringing, post-traumatic stress as a result of military service, a cyst on the brain, and a rare genetic defect: an extra Y chromosome in his genes that inclines those with this condition to violence.
The prosecution were quick to dispute the claims about his childhood and military service, casting doubts on Shawcross’ testimony. The physiological evidence about brain science and genetic factors was, at best, spurious and beyond the understanding of the jury. It was also hindered by poor presentation on the part of the expert witnesses called to testify.
Shawcross was found sane and guilty of 10 instances of second-degree murder. The judge sentenced him to 25 years for each count, a total of 250 years imprisonment. A few months later, Shawcross was taken to Wayne County to be tried for Elizabeth Gibson's murder. Rather than claim insanity this time, he pleaded guilty and received a further life sentence.
Shawcross was sent to serve his time at the Sullivan Correctional Facility in New York State.