Perhaps the most researched non-tactile form of sexual stimulation is visual sexual stimulation.[2][3][4] An apparent example is the act of voyeurism – a practice where an individual covertly watches another undress or engage in sexual behaviour. Although seen socio-historically as an unacceptable form of 'sexual deviation', it highlights the human tendency to find sexual stimulation through purely visual routes. The multibillion-dollar industry that is pornography is another example.