From the linked research paper:
Deleterious mutations are typically understood exclusively in terms of their harmful effects on carrier organisms. But there are convincing reasons to think that such adverse effects are not confined to the individual level. We argue that in social species, interorganismal gene-gene interactions, which in previous literatures have been termed social epistasis, allow genomes carrying deleterious mutations to reduce via group-level pleiotropy the fitness of others, including noncarriers. This fitness reduction occurs by way of degradation of group-level processes that optimize the reproductive ecology of a population for intergroup competition through, among other mechanisms, suppression of free-riding.
In plainspeak, the dangerously scaled-up and decadent postmodern West has lost the ability (and the willpower) to cleanse itself of toxic people who have bad DNA mojo and even worse character, and the resulting sludge will build up to the level where the West will collapse under the weight of its welfare state-supported genetic sewage (insert visual of those Ethiopians buried alive by a garbage heap avalanche (true story)).
In a way, you can view it from a greatness-reducing perspective. Suppose one person is an average guy who lucked out and became part of an 11 man team that features ten Donald Trump’s and him. There are also 11 women available, and ten of them look like Cindy Crawford, and one looks like Lena Dunham. The ten Donald Trumps get the ten Cindy Crawfords, leaving Lena for the average guy. Due to his group’s success however, everyone gets r-fied, and becomes tolerant of the treason we see typically from the fake American left.
Look at how advantageous it would be to him if he could betray his team to a bunch of mentally retarded Somali Musloids. If he could take out those Donald Trumps, and replace them with 90lb, 69 IQ Skinnies, suddenly, even though his group is now officially mentally retarded, he would have ten Cindy Crawfords to choose from as mates.
The treason of the Fake Americans is a reproductive strategy. Fake Americans, be they Cuckservatives, or their fellow anti-nationalist travelers on the left, want to create a nation of failures and the uncompetitive, because that will leave them looking quite capable by comparison.