Jeffrey Moses' defense against the accusation that he murdered his wife Jennifer was that she shot herself to death. As evidence, he wanted to call Dr. Lawrence Wilson, a forensic psychiatrist and expert on suicide.
At a pretrial evidentiary hearing, Dr. Wilson said he would testify about Jennifer's depression and substance abuse. To counter the testimony of government witnesses who said she did not appear visibly depressed, he was prepared to opine that someone who is severely depressed can mask such feelings from friends and co-workers.
As law professor Colin Marshall summarized it over at the EvidenceProf Blog:
Dr. Wilson was also prepared to testify that several risk factors, such as depression, substance abuse, and access to firearms, heighten the risk of suicide. Additionally, he was prepared to testify that lay persons do not fully understand the implications of major depression and the connection between these various risk factors and suicide. Although Dr. Wilson was not willing to opine that Jennifer Moses committed suicide, he was prepared to testify that Jennifer Moses fell "into a group of people with an extreme number of severe and significant risk factors for suicide" and that "she continued to suffer [from] major depression...that continued to the time of her death."The trial court excluded Dr. Wilson's testimony on the grounds that much of it was within the common knowledge of potential jurors, and was cumulative in light of other evidence that Jennifer did indeed suffer from depression. Also, Dr. Wilson's testimony that 15 percent of people with depression ultimately kill themselves was too prejudicial and potentially confusing to a jury, the trial court ruled.
The Washington state case is Moses v. Payne, 2008 WL 4192031 (9th Cir. 2008), available online here. The analysis by Professor Colin Miller from the John Marshall School of Law is here.