Oh criminy. That’s a model, a mathematical expression someone made up as a guess at what the real distribution might look like. In 1977, when we knew next to nothing about the actual distribution. There is no empirical data at all that says that the majority of mutations in humans are very slightly deleterious.
There are also excellent grounds for thinking that mutations of slight effect in humans are not strongly skewed toward being deleterious. For example, one class of slightly deleterious mutations are synonymous substitutions, in which a preferred codon is replaced by a slightly less efficient codon, both coding for the same amino acid. This is exactly the kind of thing you’re talking about – tiny but real effect in fitness, too small to be weeded out by purifying selection. And there are millions of sites where this could occur. So are humans experiencing a constant rain of mutations that are degrading our fitness this way? No – because human genes don’t preferentially use the more efficient codon (in technical terms, codon bias is not seen in humans). Our codon usage is already haphazard and random, and as likely to be inefficient as efficient. So synonymous changes are as likely to make things slightly better as to make them slightly worse.
The entire idea of a flood of slightly deleterious mutations reflects a significant misunderstanding of molecular evolution.
You misunderstand. By “mutations” I meant copies of mutant alleles. Realistically, each new human birth introduces roughly 2 new deleterious allele copies (effectively deleterious, in that they will eventually be removed by natural selection). A single preferential reproduction by one of the lightly loaded individuals (relative to a heavily loaded individual) removes 400 deleterious allele copies. Extrapolate that to the whole population, and you will find that there is an equilibrium mean number of deleterious alleles carried in the population for which the number removed every generation equals the number introduced.
This is all standard population genetics. The equilibrium is known as ‘mutation-selection balance’. Look it up.