Lotto players do not usually select their numbers at random. The mental process of selecting numbers is called conscious selection. In this paper, we study the importance of prime numbers in conscious selection. Our study shows that prime numbers are more popular than non primes. It follows that betting on prime numbers is suboptimal in Lotto games, which obey the parimutuel principle. We demonstrate this result on up to 10 years of data from the Belgian National Lottery (Lotto and Euromillions data). Prime numbers are selected significantly more than non prime numbers. We control for potential confounding effects, evidenced in the literature on gambling, such as the small number preference and the lucky number preference (i.e., number 7 in Western countries).