Activational effect Referring to hormonal action: the process by which hormones, by interacting with cells of the nervous system (e.g. in the brain), trigger the display of behaviours.

Affiliative behaviour Behaviour that favours association and cooperation between individuals. The expression Affinitive behaviour is also used in this context.

Allozyme Molecular variant of an enzyme.

Altruism Act of helping others at a direct cost to oneself.

Anabolic androgenic steroids Synthetic derivatives of testosterone.

Androgyny From a psychological perspective: the confluence, in the same person, of characteristics that are recognised as masculine and feminine.

Androphilia Sexual attraction towards males. The concept applies to both male and female individuals who are attracted to males.

Aneuploidy A state that results when occasionally, during meiosis, a pair of homologous chromosomes may remain together rather than dividing up and becoming part of haploid spermatozoa or ova. Whenever such chromosomally abnormal reproductive cells are involved in the production of a zygote, the resulting individual will be an aneuploid, that is, his or her cells will have an altered number of a specific chromosome.

Antecedent brother effect Developmental effect suffered especially by male offspring in association with the number of brothers born previously.

Apoptosis Programmed cell death.