Libido
Sigmund Freud popularized the term and pointed out that libido is the instinctual energy or force that can conflict with the conventions of civilized behavior.
It is the need to conform to society and control the libido, contained in what Freud defined as the Id, that leads to tension and disturbance in both society and the individual. This disturbance Freud labelled neurosis. Thus, libido has to be transformed into socially useful energy, according to Freud, through the process of "sublimation".

Libido can also be classified as the urge to copulate. For humanity, the natural way in which this occurs is through sex. Given the reason in evolutionary terms for sexual attraction and sex drive. Using this term, the antonym of libido is destrudo.

According to Swiss psychologist Carl Gustav Jung, the libido is identified as psychic energy. Duality (opposition) that creates the energy (or libido) of the psyche, which Jung asserts expresses itself only through symbols: "It is the energy that manifests itself in the life process and is perceived subjectively as striving and desire." (Ellenberger, 697)

Sometimes the sexual desire is reduced. Loss of libido may or may not correlate with infertility. Factors of reduced libido can be both psychological and physical.

Psychological factors
Reduction in libido can occur from psychological causes such as loss of privacy and/or intimacy, stress, distraction or depression. It may also derive from the presence of environmental stressors such as prolonged exposure to elevated sound levels or bright light.

Physical factors
Physical factors that can affect libido are lifestyle factors, medications and, accordingly to a study, the partner.

Lifestyle
Being very underweight, severely obese, or undernourished can cause a low libido due to disruptions in normal hormonal levels[1].

Medications
Reduced libido is also often iatrogenic and can be caused by many medications, such as hormonal contraception, SSRIs, and beta blockers. In some cases iatrogenic impotence or other sexual dysfunction can be permanent, as in PSSD.

Testosterone is the hormone behind libido in human beings. Emerging research is showing that hormonal contraception methods like "the pill" (which rely on estrogen and progesterone together) are causing low libido in females by elevating levels of Sex Hormone Binding Globulin (SHBG). SHBG binds to sex hormones, including testosterone, rendering them unavailable. Research is showing that even after ending a hormonal contraceptive method, SHBG levels remain elevated and no reliable data exists to predict when this phenomenon will diminish. Some question whether "the pill" and other hormonal methods (Depo-Provera, Norplant, etc) has permanently altered gene expression. Affected women may seek herbal and hormonal therapies. Left untreated, women with low testosterone levels will experience loss of libido, relationship stress and loss of bone and muscle and tissue mass throughout their lives. (Low testosterone may also be behind certain kinds of depression and low energy states.)

On the other hand, increased androgen steroids (e.g. testosterone) generally have a positive correlation with libido in both sexes.