Research shows that diapers and beer are the two items most commonly purchased together at the supermarket! Does that mean that having young kids in diapers drives more parents to drink? Or, that drinking beer directly causes the birth of children (in diapers)... OK, don't answer that!
History says the simian crease occurs in people with extra 21st-chromosomal material. But, there are people without Down syndrome who have the simian crease AND people with Down syndrome who do not. Are they directly related by cause and effect or do these two things happen like beer and diapers?