Betazoid

The Betazoids were a telepathic humanoid civilization originating from the planet Betazed, and were members of the United Federation of Planets.

History and politics
According to the novel The Battle of Betazed the Betazoids were finally able to drive the Jem'Hadar away by the application of a highly dangerous, offensive form of empathy. There was also a less risky but far more cruel telepathic way of handling the Jem'Hadar, but the Betazoids rejected this solution, since it would have cost them their peaceful culture.

Betazed had enjoyed a relatively untroubled history for the last few centuries. This peaceful existence came to a halt in 2374, when the Dominion invaded and occupied the planet. It was probable that the Dominion occupation ceased after the end of the Dominion War. (DS9: "In the Pale Moonlight")

Biology
Externally, Betazoids were physically indistinguishable from Humans in every aspect but one: the irises of their eyes were completely black. They could even cross-breed with Humans, along with other humanoid races like Klingons and Tavnians. Betazoid iris coloration was present in half-Betazoid individuals such as Deanna Troi, but in those with less Betazoid blood such as Devinoni Ral (who was only ¼ Betazoid), normal Human eye coloration was possible. (VOY: "Meld") Betazoids had a gestation period of ten months. (TNG: "The Child")

In mid-life, Betazoid females undergo a physiological change known as "The Phase" during which their sex drive increased by a factor of four or more. (TNG: "Manhunt")

Mature Betazoids could also suffer from Zanthi fever, which caused them to lose control over the projective aspect of their empathy. That could result in people around them acting as if they had experienced the same general emotional state as the affected Betazoid, though they were directed by their own subconscious desires, fears, and other emotions. (DS9: "Fascination")

Telepathic abilities
Betazoids were natural telepaths, an ability centered in their paracortex, with psilosynine being a main neurotransmitter. (TNG: "Dark Page") Most developed their telepathic skill in adolescence, but a few were born with their telepathic abilities already active, such as Tam Elbrun. (TNG: "Tin Man") These individuals were almost always extremely talented and powerful in telepathic terms, but also unable to screen out the noise of other people's minds, so they generally suffered mental problems of varying severity depending mostly on when the problem was diagnosed. On the other end of the scale were a few individuals who develop psionic abilities which were far below average for this species (for example, Lon Suder). Those Betazoids were barely able to sense even strong emotions (empaths) of other people, not to speak of thoughts.

The common psionic abilities of Betazoids extended from sensing thoughts and/or emotions, over projecting thoughts and/or emotions, to manipulating the minds of others. (TNG: "The Price"; DS9: "Fascination") How capable they were in performing any of those feats depends somewhat on their genetically defined psionic strength, their psionic training, (TNG: "Haven") their familiarity with the scanned being, their general mental and physical condition and the species of the subject race. Betazoids had been able to read the emotions/thoughts of non-corporeal beings such as Nagilum, the energy vortex, or gauge the mental prowess of beings such as Q. Other times they had been totally unable to read corporeal creature's minds, such as Ferengi and Changelings. Data theorized that the reason they may not be able to read Ferengi was the structure of their brains. And he himself was able to be read by Deanna Troi when his brother Lore used the emotion chip to transfer powerful emotions to him. At one point, when Deanna Troi lost her empathy temporarily she commented that Will Riker was like a holodeck character to her, completely devoid of emotion, as if she had never experienced a lack of empathy from any sentient before. The Traveler was also not able to be read by Deanna Troi as well.

Inter-species reproduction involving Betazoids often affected the psionic abilities of the offspring – most commonly the children of such a union develop empathic abilities as their primary psionic talent, while their telepathic abilities, though existing, were rather below average for Betazoids. Usually the telepathy of these half-breeds, without extensive training, was limited to communication with other empaths or telepaths and full telepathic contact with emotionally very close persons (for example, an imzadi). All full Betazoids are unable to read the thoughts of Ferengi, Breen, Ullians, or Dopterians, but some half-Betazoids have been occasionally successful in sensing the emotions of some of these species. Additionally, even full Betazoids seemed to be able to psionically influence some of these species. (TNG: "The Price", "Ménage à Troi"; DS9: "The Forsaken", "Fascination")

According to the novel The Battle of Betazed, some hidden mental teachings are able to unlock the ability of telekinesis in Betazoids.

Culture and tradition
Due to their telepathy, Betazoid culture embraced honesty almost to a point considered rude by other cultures. Lwaxana Troi, on numerous occasions, commented on her befuddlement at the Human practice of fibbing to spare others' feelings or for politeness' sake. (TNG: "Haven", "Dark Page")

Betazed had a complex hereditary nobility; prominent Betazoid diplomat Lwaxana Troi, for example, was "Daughter of the Fifth House, Holder of the Sacred Chalice of Rixx, and Heir to the Holy Rings of Betazed." Betazoid tradition had children genetically bonded to a future spouse. (TNG: "Haven")

In the Betazoid wedding ceremony, all participants are traditionally nude, not only the bride and groom, but the guests as well. (TNG: "Haven", "Cost of Living"; Star Trek Nemesis)

At one time, it was fashionable for Betazoid women to wear elaborate wigs that caged small animals. The practice, which was cruel to the animals, was stopped when one (unknown) woman stood up against it. (TNG: "Half a Life")

It had been speculated that Betazoid society in the past was matriarchal. (TNG: "Angel One")

Their religious belief system may involve polytheism because in one instance, Lwaxana Troi used the exclamation, "Thank the Four Deities". (TNG: "Manhunt")