Intelligence

Intelligence is one of seven Primary Statistics in the SPECIAL character system.

Fallout, Fallout 2 and Fallout Tactics
Modifies: the number of new Skill Points per level, dialogue options, and many skills.

One of the unique qualities of Fallout and Fallout 2 was the difference in gameplay caused by creating a low Intelligence character. If you have a low Intelligence, many non-player characters will simply shrug you off, while others will insult or even attack you. It is possible to complete the game as an idiot, but most side-quests are impossible to do, and character growth is limited, as is the story involvement possible - while you can certainly bumble through the main quests, almost all plot details and background information will be left missing. Furthermore, due to limited-at-best non-player character interaction, almost all side-quests are unavailable. Because of this, playing as a low Intelligence character is only recommended for experienced players looking to role-play, or as a humorous replay option.

Intelligence can be raised permanently in Fallout 1 by Lorri's implants and in Fallout 2 using ACE and the Yellow memory module (+1) and by the Hubologist zeta scan in San Francisco (also +1 Luck, though you may have to save/reload to get a bonus rather than a penalty). Mentats give a temporary +2 Intelligence.

Fallout 3
Modifies: Medicine, Repair, and Science skills, as well as the number of new Skill Points per level.

Intelligence determines the number of skill points you receive each time you level up. The base number of skill points gained per level is 10+Intelligence. Skill points gained for raised Intelligence are not retroactive for past levels, so increasing this primary stat early if at all is the best. Also, temporary modifiers to Intelligence (e.g. from Night Person or Button's wig) do not affect the number of skill points gained.

Ways to increase Intelligence

 * The Bobblehead - Intelligence is in the Rivet City Lab.
 * The Night Person Perk increases your Perception and Intelligence when the sun is down.
 * The Intense Training perk will allow you to increase your Intelligence (or any other S.P.E.C.I.A.L. stat) once for each time that you take it.
 * Lincoln's hat and Button's wig each give +1 to the stat. This does not affect skill points gained at level up, but does increase the relevant skills.
 * Mentats gives a temporary +5 to the stat. This does not affect skill points gained at level up, but does increase the relevant skills.

Fallout: New Vegas
Intelligence affects skill points differently than it did in Fallout 3. The number of skill points gained each level is based on the formula 10 + (0.5 * INT) per level, including level 1, where INT is your Intelligence. If you get a non-whole number of skill points per level (e.g. 10.5 for INT 1) it will be saved for the next even level up. (See Skill Rate)

Ways to increase Intelligence

 * The player will gain +2 Intelligence and +2 Perception when the sun is down with the Night Person perk.
 * The player can purchase an Intelligence Implant from the New Vegas Medical Clinic for 4000 caps. This raises Intelligence by +1, unless the player already has 10. If the player's Intelligence is low enough, they will only have to pay 3000 caps.
 * Mentats gives a temporary +2 to the stat. This does not affect skill points gained at level up, but does increase the relevant skills.
 * Party Time Mentats also add +2, and stack with regular Mentats (and vice versa) giving a temporary +4 increase. (e.g. Intelligence set at 6 can reach 10, giving 8 skill points towards Medicine, Science and Repair.)
 * Black coffee adds a temporary increase of +2, +4 or +6, increasing relative to the player's Survival skill.
 * Dr. Mobius' glasses add a temporary bonus of +2

Intelligence in quests and exploration

 * If the player has an Intelligence of 1 after choosing their SPECIAL stats at the beginning of the game, Doc Mitchell will say "Sorry, son/little lady. I fixed up your head as best I knew how. Guess I missed a spot."
 * When speaking with Arcade Gannon, if you have Intelligence of 3 or lower you can convince him to become your companion by stating "This place is really complicated and stuff. You're smart, can you help me do... stuff?".
 * If creating a character with Intelligence of 3 or less can grant 'special' Intelligence checks. If the player's Intelligence is really low, dialog options will appear to be short and monosyllabic, with barely any sentience.
 * When first meeting Ignacio Rivas at Helios One, he will ask the player what their interest in the place is. If the player's Intelligence is high, they may respond with "My interest in this place is scientific." However with a low-Intelligence player, the same option will be available as "I is scientistic."
 * If the player has a low enough Intelligence, they will admit they don't know what a fish is during a conversation with Cass.
 * An Intelligence of 6 is needed for Dr. Mobius to reveal that they are still planning how to save themselves.
 * An Intelligence of 8 is needed to reveal the fact that Dr. Mobius scattered the personality chips across Big MT.

Fallout 1 & 2
When the Player Character in Fallout and Fallout 2 has less than 4 Intelligence, the biggest change in gameplay is undoubtedly the dialogue. Upon reaching Shady Sands, the villagers take pity on you, and Aradesh just shrugs you off. The best you can speak is in half-word sentences or gibberish.

The real downside, though, is that you have no chance to do about 90% of the side quests. No one will talk to you or even give you the time of day (Even though you have it on your Pip-Boy).

In Fallout 2, you may meet Torr, the "town simpleton of Klamath", who would normally give you a quest to under go protection of his brahmin against "the evil bugmen". He will appear highly intelligent to you, as if being stupid is a culture and language all its own, and two unintelligent people can have a complete, thought-provoking, and fully articulated conversation between them.

Another person you may have an "intelligent" conversation with in Fallout 2 is AHS-9, who becomes able to understand you after you become aligned and gives you the quest to Kill the Shi Emperor.

When talking to Algernon in the basement of New Reno Arms you can start a "childhood cop and robbers" type dialog in which you accidentally shoot and kill him.

Fallout 3
In Fallout 3, there are conversation situations where characters with low Intelligence are granted other speech options, often relating to their low intellect. However, these options are few and far between, unlike the other Fallout games where one could play the entire game as a person with low Intelligence. One instance is in Roosevelt Academy, where if you activate Dean Dewey when you have an Intelligence of 3 or lower he will deem you a "special needs" child, and escort you to a 'proper room'.

Fallout: New Vegas
In addition to alternative Intelligence checks, low Intelligence changes some dialog options when speaking to vendors and non-player characters. For example, when speaking to Miguel at Miguel's Pawn Shop, the option to start trading is "Me buy things", and when speaking with Dr. Usanagi at the New Vegas Medical Clinic, after she gives you the initial description about implants you have the dialogue option "So you sell plants?", which will cause her to re-explain the implants and suggest you take the implant to make you smarter. If you take her up on this deal she will state that she thinks "you could really benefit from it", and offer the implant at a discount.

In place of the gibberish of past games, however, the player will simply not completely understand a conversation, confuse what someone is saying with something else entirely, or not understand what situation they are currently in. In the REPCONN headquarters, for example, a character with an Intelligence of 2 or less can bypass a robot that demands a password by randomly shouting "ICE CREAM!". Normally, the player would need a Luck of at least 7 to guess this.

Low Intelligence characters attempting to do the That Lucky Old Sun quest will be granted several unique and highly entertaining dialog options when conversing with Fantastic and Ignacio Rivas. An example is when informed about the hostilities between the Followers and the NCR, the player can choose to respond, "Do Flowers and NCR bear play together?"

Fallout 4
The Idiot Savant perk means low Intelligence builds can compete with high Intelligence builds in earning experience points by randomly multiplying XP earned from various sources, with a greater chance at lower Intelligence.

Fallout 4
With the absence of individual skills, Intelligence improves the amount of experience points players receive from all sources.

Ways to increase Intelligence

 * Permanent
 * Night Person perk rank 2 (+3, but only between 6PM and 6AM)
 * Night Person perk rank 1 (+2, but only between 6PM and 6AM)
 * Intelligence bobblehead (+1)
 * Intelligence Training (+1)
 * You're SPECIAL! book (+1)
 * Temporary
 * Armor and clothing
 * Liam's glasses (+2)
 * Vault-Tec lab coat (+2)
 * Lab coat (+1)
 * Nuka-World jumpsuit (+1)
 * Ushanka hat (+1)
 * Chems
 * Berry Mentats (+5)
 * Mentats (+2)
 * X-Cell (+2)
 * Power armor modifications
 * Internal database helmet mod (+2)

Fallout: New Vegas

 * Me help shoulders make shinies warm. - the Courier
 * I IS SCIENTISTIC! - the Courier
 * What are Flowers of Pock-lips? - the Courier
 * Ooh Shiney Robot - the Lone Wanderer
 * So you sell plants? - the Courier
 * ICE CREAM! - the Courier
 * Me pet aminals - the Courier
 * I no hurt anyone - the Courier
 * Where I send shiny juice? - the Courier
 * Me take you job cause me is smarter - the Courier

Lionheart
The higher the character’s Intelligence, the more skills the character will excel in, and the more options he will have in dialogue. The number of skill points the character receives at each experience level is based on the character’s Intelligence.