Review Article

Why infest the loved ones – inherent human behaviour indicates former mutualism with head lice


a1 MTA-ELTE-MTM Ecology Research Group, Budapest, Pazmany Str. 1, H-1117 Hungary

a2 Department of Plant Taxonomy and Ecology, Eotvos University, Budapest, Pazmany Str. 1, H-1117 Hungary


Head lice transmit to new hosts when people lean their heads together. Humans frequently touch their heads to express friendship or love, while this behaviour is absent in apes. We hypothesize that this behaviour was adaptive because it enabled people to acquire head lice infestations as early as possible to provoke an immune response effective against both head lice and body lice throughout the subsequent periods of their life. This cross-immunity could provide some defence against the body-louse-borne lethal diseases like epidemic typhus, trench fever, relapsing fever and the classical plague. Thus the human ‘touching heads’ behaviour probably acts as an inherent and unconscious ‘vaccination’ against body lice to reduce the threat exposed by the pathogens they may transmit. Recently, the eradication of body-louse-borne diseases rendered the transmission of head lice a maladaptive, though still widespread, behaviour in developed societies.

(Received July 12 2011)

(Revised September 20 2011)

(Revised December 16 2011)

(Revised January 02 2012)

(Accepted January 03 2012)

(Online publication February 06 2012)


c1 Corresponding author: MTA-ELTE-MTM Ecology Research Group, Budapest, Pazmany Str. 1, H-1117 Hungary. Tel: +36 1306957185. E-mail: