The claims made about botanical sexism have been contested. Rita Sousa-Silva, an assistant professor of ecology at the University of Leiden, has said that the OPALS rating system has "no scientific background". Some have pointed out that the majority of tree species (~75% globally) are cosexual, meaning they produce flowers with both male and female parts. As a result, a human sexual binary does not apply to most trees, with only 5% globally being dioecious, though this does not preclude the possibility that urban trees could be largely male. William Elmendorf, a professor of urban forestry management at Penn State University, has said that terms like "fruitless" or "podless" were previously used more commonly to refer to dioecious trees selected for low fruit production, such as ginkgos. It has been noted that there is limited data to confirm or deny the claim that male trees are more prevalent than female trees in the urban landscape. The idea that additional female trees would significantly reduce pollen has also been challenged with some instead pointing to planting fewer wind-pollinated species.