Just as creatures of the Otherworld have a response to our reality in the form of The Allergy, people of our reality have a response to the Otherworld in the form of The Lust. This has not been observed in non-humans while on expedition or in folkloric reports and this delicate subject matter has not been put to rigorous scientific study and remains effectively anecdotal only. This is unfortunate because it is currently one of the few means by which the Otherworld boundary can be surely mapped.
The Lust response exists in a spectrum and those who are most sensitive to the presence of the Otherworld, i.e. have the most Lustful response, are said to make the best Wardens, who monitor the location and strength of the boundaries. While Wardens are also supplied with equipment that can take various measurements of the reality fields, these cannot detect those places where a crossing is possible with the same surety. Wardens tend to work on their own or in bonded pairs in order to reduce the embarrassment Otherworld-exaggerated appetites can cause.
Rangers, who are expected to cross into the Otherworld on a regular basis, under go severe training in order to be able to control their appetites while in larger groups and on expeditions with outsiders. However, the presence of The Lust has contributed to the use of the Ranger stereotype in romance and erotica.
The presence of The Lust may be responsible for the ease of which ballad heroines, such as Margaret or Janet in Tam Lin, accept the advances of the fairy hero.