The phrase goes back to the 1600’s, predating the field of genetics by almost 300 years.
The idea that how we act is literally running through our veins is often seen in expressions of negative emotions like animosity (bad blood), anger (my blood is boiling.), fear (blood run cold), cruel (cold blooded) and vengeful (out for blood).
Beyond colorful idioms, there is more truth to the idea than we may realize.
Consider this study on violence summarized in Robert Sapolsky’s book Behave. Undergraduate male subjects had a blood sample taken, filled out a brief questionnaire and then walked down a narrow hallway to drop them off – where a second sample was taken. For half of the subjects, a burly man walking in the opposite direction would bump into them and call the subject an a—hole. The other half had an unimpeded walk. The differences were extraordinary.
Subjects from the south who were insulted had massive increases in the stress hormone cortisol and testosterone levels (more than 300% vs. control.) while those from the North remained largely the same. The hypothesis was that those from the South have a long tradition of a more pastoral culture dating back to herders who emigrated from Scotland, where honor codes were sacred.
Insults like those in the study are an affront to their honor and the visceral and physical response is a result of “who they are” culturally.
This also may explain why the majority of southern homicides are argument-related murders with people they know vs. felony related murders that are the norm in Northern states.
We like to think that who we are and how we act are always of our own volition. The reality is, of course, much more complicated – giving new meaning to the term “blood-type.”