“Unlike the science of medicine generally, psychology does not lend itself to hard rules well.”
As someone who has written dozens of peer-reviewed scientific papers analyzing data from psychological studies, I have to say that you are wrong. Measurements of behavior are often as reliable as any other in medicine, and observable behavioral measures are now correlated with biomarkers from things like brain images. This has been seen in many studies, from with memory and behavioral changes in elderly persons developing dementia to biological changes in person suffering from traumatic stress (including holocaust survivors).