What impact should biological tests have on the sentencing of criminals?

In the August 17th edition of Science Magazine an article reports on an interesting study done by the University of Utah in Salt Lake City. In the study, 181 state trial judges were asked to review the details of mock case and were asked to imagine how they would rule in a sentencing hearing.  In the case, the imaginary Jonathan Donahue is convicted of beating the manager of a fast food restaurant so badly that he is left brain damages. The details of the case make it clear that Mr. Donahue is a textbook psychopath. All of the judges who participated in the study read testimony from a psychiatrist who testified that Donahue had been diagnosed as a psychopath. However, some of these judges also read testimony from a neurobiologist who presents results of a genetic test that Donahue possessed a specific genetic variant linked to violent behavior.

The judges who read this biological explanation of why Donahue may have behaved in this way handed out moderately smaller sentences, 13 years, as compared with judges who only saw the testimony from the psycholgist, who game Donahue 14 years. The authors of this study concluded that the introduction of expert testimony giving a biological explanation for behaviors can be a "double-edged sword in the courtroom."  They could either be seen as evidence that a defendant is hard-wired for bad behavior and destined to re-offend, or it could be seen as evidence that the defendant's behavior was out of his control.

Should genetic tests and brain scans that show biological factors for individuals' behavior be allowed as evidence in sentencing hearings of this kind?

In Mock Case, Biological Evidence Reduces Sentences by Greg Miller. Science Magazine. Vol. 377, no. 6096. p. 788.