well as some other non-gender features like animate, edible, manufactured, and so forth. An example of the latter is found in the Dyirbal language. Other gender systems exist with no distinction between masculine and feminine; examples include a distinction between animate and inanimate things, which is common to, amongst others, Ojibwe, Basque and Hittite; and systems distinguishing between people (whether human or divine) and everything else, which are found in the Dravidian languages and Sumerian. A sample of the World Atlas of Language Structures by Greville G Corbett found that fewer than half of the 258 languages sampled have any system of grammatical gender. Of the remaining languages that feature grammatical gender, over half have more than the minimum requirement of two genders. Grammatical gender may be based on biological sex (which is the most common basis for grammatical gender), animacy, or other features, and may be based on a combination of these classes. One of the four genders of the Dyirbal language consists mainly of fruit and vegetables. Languages of the Niger-Congo language family can have as many as twenty genders, including plants, places, and shapes. Many languages include terms that are used asymmetrically in reference to men and women. Concern that current language may be biased in favor of men has led some authors in recent times to argue for the use of a more gender-neutral vocabulary in English and other languages. Several languages attest the use of different vocabulary by men and women, to differing degrees. See, for instance, Gender differences in Japanese. The oldest documented language, Sumerian, records a distinctive sub-language only used by female speakers. Conversely, many Indigenous Australian languages have distinctive registers with a limited lexicon used by men in the presence of their mothers-in-law (see Avoidance speech). As well, quite a few sign languages have a gendered distinction due to boarding schools segregated by gender, such as Irish Sign Language. Several languages such as Persian or Hungarian are gender-neutral. In Persian the same word is used in reference to men and women. Verbs, adjectives and nouns are not gendered. (See Gender-neutrality in genderless languages). Several languages employ different ways to refer to people where there are three or more genders, such as Navajo or Ojibwe. Science Historically, science has been portrayed as a masculine pursuit in which women have faced significant barriers to participate. Even after universities began admitting women in the 19th