This hypothesis is called the domestication syndrome, and its existence was challenged in 2019. It was not known what principle of selection had guided the Neolithic farmers who had first domesticated these species thousands of years ago. Belyayev's hypothesis was that "all domesticated species had been selected for a single criterion: tameness."
This syndrome causes increased hyper-sociability, which may have been important during domestication.  In 2014, a whole genome study of the DNA differences between wolves and dogs found that dogs did not show a reduced fear response but did show greater synaptic plasticity .
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SOURCES: Science: "Starch fossils and the domestication and dispersal of chili peppers (Capsicum spp. L.) in the Americas."Adrienne Youdim, MD, associate clinical professor of medicine, UCLA David ...