Psychic has a natural resistance against fighting, and fighting is incredibly weak for the psychic. Psychic also resists its own kind, and psychic attacks hit incredibly well against Poison. When entering into battle against Psychic-type Pokémon, it's worth knowing what their weaknesses are. Psychics are weak against Bug, Ghost, and Dark-type Pokémon.
Dark types are especially useful, since they are immune to psychic movements. The Psychic type has few absolute strengths, however, he also has few weaknesses. In the first generation it ended up being massively dominated, mainly due to the complete lack of powerful movements of insects, its only weakness. In addition, a bug in the game meant that Ghost-type moves had no effect on Psychic (although this only affected the low-powered Lick).
Generation 2 corrected the situation with the addition of the Dark type along with better Pokémon and moves of all kinds. This is not to say that psychics were unopposed in Pokémon Red and Blue. There were some incredibly powerful normal guys in this generation, such as Chansey, Snorlax and Tauros, who also benefited from the primitive combat system and broken movements. However, the writing was on the wall, since the psychics were too strong.
Game Freak added two new types in Pokémon Gold and Silver, in the form of Dark-type and Steel-Type. Dark-type Pokémon were totally immune to Psychic-type movements, while Steel-type Pokémon resisted them. Ghost-type moves have also changed their effectiveness, and now deal super-effective damage to Psychic-type Pokémon. These changes greatly weakened the Psychic types in Generation 2, and the guy has never again reached the level of power he had in Pokémon Red and Blue.
However, in the days of Pokémon Red and Blue, the Psychic type was the favorite, being so overwhelmingly powerful that the next set of games had to introduce new types to balance it. Of course, this also means that it makes little sense to bring a Psychic Pokémon to fight another Psychic Pokémon. Psychic-type Pokémon take more damage from insect, ghost, dark moves, and less damage from combat, psychic-type moves in Brillaint Diamond and Shining Pearl. Poison-type Pokémon are also vulnerable to psychic movement, so you might leave them inside the Pokeball when you're in battle.
The fraction of damage a psychic-type move will inflict is shown: ½ means 50% damage (not very effective), 2 means 200% (super effective), etc. Psychic damage is not elemental, but Psychic-type Pokémon can also use elemental moves along with their psychic moves. Pokémon Weakness Guide of Pokémon Shining Diamond and Shining Pearl Psychic-Type, below you will find a psychic-type movement damage effectiveness graph. A psychic-type Pokémon is any Pokémon that specializes in certain moves that use psychic themes to damage or inflict status effects on an opponent.