The Brexit referendum and the US presidential election of Donald Trump were a surprise to many, on both sides of the fence. In this information age, it is useful to ask how so many people could be so wrong about issues of so much importance. If we all knew what everyone else was thinking there could be no surprise. We are, in fact, wrong about many things when it comes to estimating the beliefs of the majority. Many of our errors arise not because our brains are tricking us, but because our appreciation of structure is underdeveloped. For example, for the majority of people, the places they vacation to are not average destinations, just as the traffic they experience is not average traffic nor the classes they sit through of average attendance. This is because, by definition, the most crowded places are attended by the most people. Similar illusions lead us to overestimate how many friends the average person has, confuse us into making backwards inferences about class and gender divisions, and allow politicians to misrepresent their populations. All of these are guaranteed outcomes of certain kinds of structure which an understanding of networks demystifies.

● Darwin invented the concept of group selection to explain the evolution of traits that lead individuals to improve the fitnesses of others at a fitness cost to self. Such traits are now called “altruistic.” ● Understanding Simpson’s paradox is key to understanding how natural selection can cause altruism to increase in frequency in a meta-population. ● A criterion is derived for when altruism is fitter than selfishness in a meta-population in which there are groups of size 2. The relevance of correlation and genealogical relatedness to the evolution of altruism is discussed, as is the question of whether reciprocal altruism is really a form of selfishness. ● The concepts of cultural group selection and species selection require further refinements in how group fitness needs to be understood. ● In addition to individual selection and group selection, there is a third unit of selection – intragenomic conflict. Meiotic drive is a classic example. ● The reductionist thesis that group and individual selection reduce to selection on genes is criticized, as are conventionalist theses that assert that it is a matter of convenience, not biological fact, whether group selection occurs in a population.