I remember a long time ago (perhaps 20 years ago) that an elementary school textbook on health said "mixing alcohol beverages exacerbates the strength of the alcohol in the drink, thereby potentially being fatal to those who drink mixed beverages."
Is this actually true? To my knowledge, basic chemistry dictates that you cannot have more concentration of an element than there was before you mixed the two beverages; for instance, if you mix 100ml of beer(4.5%) and 10ml of spirits(20%), you will only have a 110ml of mixed beverage with 6.5ml of alcohol(4.5%*100+20%*10). In other words, you will have a 110ml(5.9%) drink. Which can't be that much more dangerous than drinking 100ml of beer(4.5%).
So maybe I am remembering the textbook wrongly -- maybe it said that mixing drinks leads you to drink more alcohol? Or is it true that there is a third effect going on -- maybe mixing two different drinks leads to a new, more stronger substance that is greater than the mix of the two drinks? Perhaps because different drinks have different kinds of ethanol mixed into them such that when you combine the two there is a sort of alchemy that makes two seemingly harmless drinks into a sip of poison?