The experience of a previous conflict can affect animals' performance during a later contest: a victory usually increases and a defeat usually decreases the probability of winning a subsequent conflict. These winner and loser effects could result from a reassessment by contestants of their perceived fighting abilities. Game-theoretic models based on this assumption predict that a loser effect can exist alone or in the presence of a winner effect, but a winner effect cannot persist alone, at least when contestants are young and without experience of contest. Moreover, when both effects coexist, the loser effect is expected to be of a greater magnitude and last longer than the winner effect. To date, these predictions have been supported by empirical evidence. Here we show for the first time that a winner effect can exist in the absence of any evident loser effect in a parasitoid wasp, Eupelmus vuilleti, when fighting for hosts. This finding consequently raises questions about the possible mechanisms involved and challenges the main assumption of previous theoretical models. We suggest an alternative explanation for the evolution of only winner effects that is based on the modification of contestants' subjective value of the resource rather than on a reestimation of their fighting abilities.