Then why are there studies that show that children of mixed races sometimes get "hybrid vigor" or heterosis, in this case they get taller than both parents, and get a higher measured IQ than any of the parents? https://socgen.ucla.edu/2015/07/02/diverse-parental-genes-lead-to-taller-smarter-children-finds-extensive-study/
That study doesn’t claim that mixed race individuals are superior. Indeed, one could argue it’s an argument against European and Asian people procreating with those of other races. The study measured zygosity and found that “increased homozygosity was associated with decreased trait value.” Homozygosity is not a measure of whether someone is mixed race. It’s more likely a proxy for whether a given population is inbred (which itself associates with whether a population consistently engages in practices such as marrying a 12 year old to her 50 year old uncle).
Of all racial groups for which data is available, Europeans and Asians are among the least inbred and so the least likely to have adverse homozygosity:
Mixed-race children are more obese than their parents relative to non-mixed children. Obesity indicates metabolic dysfunction. Monoracial children, having better metabolism, would be preferable.
Mixed-race children also have more health problems and have worse mental health.
Mixed-race adolescents also are more likely to exhibit antisocial behavior.