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Libido
Libido in its common usage means sexual desire; however, more technical definitions, such as those found in the work of Carl Jung, are more general, referring to libido as the free creative—or psychic—energy an individual has to put toward personal development or individuation. Sexual desire was originally for animals to reproduce, but people are using it for their pleasure. Libido tends to be very strong in males of all species and tends to have a low threshold for its expression. Within the category of sexual behavior, libido would fall under the appetitive phase where a male or female will usually undergo certain behaviors in order to gain access to the opposite sex.
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Psychological and Physical Factors