In the UK if it is evidence to a crime, Oh yes.
Much too general a question to answer. So much depends upon the type of case, and the prevailing 'rules of evidence.'
To start, much of the evidence must be backed up with factual evidence, so that the circumstancial evidence is minimal.
In most states, no. However, in some states they are allowed under certain circumstances. It's typically not within the trial judge's discretion when to and not to admit this evidence. Most states'...
Hearsay