Hangovers are caused by acetaldehyde and some dehydration. Human biology gets rid of alcohol(ethanol, CH3CH2OH) by first converting it into acetaldehyde (CH3CHO) and then vinegar (CH3COOH), which is very quickly mixed with water into urine by your kidneys. Over the last few thousand years, humans have actually evolved to have worse hangovers. Acetaldehyde intoxication is unpleasant, but not more damaging than alcohol, so poeple who get rid of it slowly consume less alcohol and thus are healthier overall and have healthier children.