42 Pa.C.S.A. is a relatively new law which allows the Commonwealth to introduce expert testimony related to so called "victim response experts" to testify as to rape trauma syndrome and responses to sexual violence. The testimony is supposed to be general and not specific to the particular case (although it rarely is, more on that later). The law was intended as a way for the Commonwealth to "educate" regular citizens about how someone might respond to sexual violence.
The problem with this law is that the field of rape trauma syndrome (RTS) is junk science. It is qualitative, not quantitative, meaning that is does not rely on actual statistics or numbers. It is not useful as a prediction tool and cannot be used to determine the truth or falsity of an allegation of sexual violence.
An excellent recent article on this is Examining the Scientific Validity of Rape Trauma Syndrome, published this past July in Psychiatry, Psychology and Law. The authors there tear apart the studies used by the RTS field and show how unscientific the field really is.
An additional issue with the law is that it was passed by the Legislature to attempt to create a new rule of criminal procedure. The Pennsylvania Constitution's Separation of Powers doctrine forbids exactly that - the rules of procedure of the Courts are the Courts' exclusive domain. The law is therefore also unconstitutional in addition to being based on bad science.
A court out of Berks County recently held the law unconstitutional. That decision has been challenged and the matter is currently before the PA Supreme Court. I am hopeful that the law will be ruled unconstitutional and overridden. However, even if it is ruled unconstitutional that would not necessarily prevent the Court from promulgating such a rule on its own. The Court should review the scientific literature which holds rape trauma syndrome to be politically correct nonsense and disallow the use of such testimony.
In doing so the Court would be upholding decades of precedent which held exactly that and prohibited this kind of grandstanding and back door credibility boosting by the Commonwealth.