The influence of the Church most likely loosened the clan structure and warfare among the Germanic people. However, that cousin marriage leads to lower intelligence is, so far, a claim not backed up by any credible evidence.
Apart from individual cases—Kierkegaard, Darwin, Einstein,
etc.—the following comments by Dr. Michael A. Woodley are of
interest:
Are We Getting
Dumber? Dr Michael A. Woodley and Stefan Molyneux [YouTube.com].
See also “HBD” is globalism.