Medical journals ? Hmmm .You don't need to be a scientist to figure this out. Interbreeding causes the limiting of the gene pool that the offspring will receive. As a result the offspring has a much higher chance of developing recessive disorders (which occur due to a small gene pool).
The years of medical research might be wrong but here's a fun project you can try at home. Start with two plants (any species) and have their offsprings self pollinate over 3 or 4 generations and you'll see it with your own eyes that over time the offspring will be weaker or might not grow at all. Not everything is a hinjew saazish against us, legitimate science exists.