01011nas a2200133   4500000000100000000000100001008004100002100002600043700002700069245009000096300001200186490000700198520067200205       2008                            d1 aJosé Aizpurua Agirre1 aLoreto Erviti Llorente00aA Betting Market: Description and a theoretical explanation of bets in Pelota Matches  a421-4460 v643 aIn Pelota matches, bets are made between viewers through a middleman who receives 16\% of the finally paid amount. In this paper, a description of the way bets are made and an explanation of the existence of those markets are presented. Taking betting markets as a simplified analogy for financial markets we have searched for the explanation in a world where both sides of the market are not different in believes and preferences. We find that for a bet to exist when bettors are rank dependent expected utility maximizers, they have to be optimistic. Taking observations from actually made bets a preliminary analysis about the biases of those markets is presented.