Research on the etiology of sexual orientation when applied to asexuality has the definitional problem of sexual orientation not consistently being defined by researchers as including asexuality. Sexual orientation is defined as "enduring" and resistant to change, proving to be generally impervious to interventions intended to change it, and asexuality may be defined as a sexual orientation because it is enduring and consistent over time. While heterosexuality, homosexuality and bisexuality are usually, but not always, determined during the early years of preadolescent life, it is not known when asexuality is determined. "It is unclear whether these characteristics ["viz.", "lacking interest in or desire for sex"] are thought to be lifelong, or if they may be acquired."