Consider a group of individuals who form a social network. For each individual in the group compute its friendship-bias, i.e., the difference between the average number of friends of its friends and the number of its friends (all friendships are mutual), and average these numbers over all the individuals in the group. It turns out that the latter average is always non-negative, and is strictly positive as soon as not all individuals have exactly the same number of friends. This fact, which at first glance seems counterintuitive, goes under the name of friendship paradox.

Based on joint work with Rajat Hazra (Leiden), Nelly Litvak (Eindhoven) and Azadeh Parvaneh (Leiden).