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Still, despite peer-reviewed evidence, many parents ignore the math and agonize about whether to vaccinate. Why? For starters, the human brain has a natural tendency to pattern-match — to ignore the old dictum “correlation does not imply causation” and stubbornly persist in associating proximate phenomena. If two things coexist, the brain often tells us, they must be related. Some parents of autistic children noticed that their child’s condition began to appear shortly after a vaccination. The conclusion: “The vaccine must have caused the autism.” Sounds reasonable, even though, as many scientists have noted, it has long been known that autism and other neurological impairments often become evident at or around the age of 18 to 24 months, which just happens to be the same time children receive multiple vaccinations. Correlation, perhaps. But not causation, as studies have shown.