Lady Killer

 or Lady Killer (depending on the Player Character sex) are perks in Fallout 3 and Fallout: New Vegas.

Fallout 3
In combat, you do +10% damage against opposite sex opponents. Outside of combat, you'll sometimes have access to unique dialogue options when dealing with the opposite sex (the latter is much like the Sex Appeal Trait). Both perks' images feature a female character who can be construed to be the female analogue of the Vault Boy.

Black Widow

 * You can talk Ronald Laren in the Nuka-Cola Challenge quest into a threesome. He then runs to the factory himself. He always seems to end up dead just inside the door to the Nuka-Cola factory if you do this, even if you accompany him all the way there; if he is still alive when the two of you arrive, he will just wait in the lobby while you explore the factory alone. When you come back to the entrance, he will be dead.
 * You can talk Burke in Megaton into paying you double to do the job, or to give up on the bomb entirely.
 * You can talk Karl (during the Blood Ties quest) into giving you the password to Ian West's room.
 * You can talk The Mechanist into stop fighting his nemesis, the AntAgonizer. You also get his costume.
 * You can talk Dukov out of his key without paying for it for the You Gotta Shoot 'Em in the Head quest.
 * You can talk Uncle Roe into organizing the trade caravans.
 * You can convince Ramsey into selling slaves to you during Into the Pitt.
 * You can talk with Defender Morrill at the beginning of Operation Anchorage, so you can come through after the elevator, but that should not be a problem even if you don't have the Black Widow perk.
 * You can convince Wernher to give you 120 5.56mm Rounds should you decide to side with the slaves during Free Labor.
 * You can use this perk on Sibley in the beginning of the Operation: Anchorage quest-line, where he will lead you to Protecter McGraw, without the aggression. He still turns on you at the end, though.
 * You can use this perk on Paulson when you first meet him during Mothership Zeta. He will tell you why he wants revenge.

Lady Killer

 * Nova in Megaton will provide you with the password to Moriarty's terminal.
 * Brianna in the Blood Ties quest will provide you with the password to Ian West's cell.
 * You can talk the AntAgonizer into stopping her attacks on the town and fighting The Mechanist. You also get her costume.
 * You can talk Fantasia into stealing Dukov's key for the You Gotta Shoot 'Em in the Head quest.
 * You can talk Agatha into giving you ammo to help you during the Agatha's Song quest.
 * In Operation: Anchorage, you can calm down Specialist Olin and get more detailed information on questions. Also, the conversation will not end after each question.

Fallout: New Vegas
Both Black Widow and Lady Killer re-appear in Fallout: New Vegas, supplemented by Cherchez La Femme and Confirmed Bachelor.

Lady Killer

 * Lady Killer: You can flatter Old Lady Gibson to give you the thruster mods for free during the "Come Fly With Me" quest.
 * Lady Killer: You can make Janet immediately fall in love with Jack during the "Young Hearts" quest.
 * Lady Killer: A hooker named Dazzle in the courtyard of Gomorrah will have sex with you for 50 caps instead of 100. However, it should be noted that she'll have sex with anyone for a Med-X if they can make a Speech 35 or Medicine 35 success.
 * Lady Killer: You can flirt with Lady Jane to have her pay her debt to the Atomic Wrangler
 * Lady Killer: You can invoke some humorous dialog between you and the Lonesome drifter; he informs you that he lived in Montana and his father left before he was born, the player has the choice of worriedly asking "Montana? Oh god, you wouldn't happen to be 17 would you?" to which he replies in the negative, and the player responds by thanking God and attempting to change the subject.

Black Widow

 * Black Widow: You can convince Chris Haversam that he is indeed human, thus allowing you to convince him to sabotage the rockets for the quest Come Fly With Me.
 * Black Widow: After asking for some work, you can persuade Swank to give you some caps to buy yourself something pretty.
 * Black Widow: You can befriend Benny and get him to have sex with you when you first meet him, leading to a chance to kill him quietly in the night.
 * Black Widow: You can convince Old Ben to take up the escorting business again in Wang Dang Atomic Tango.
 * Black Widow: You can flirt with Jack at the Nellis Air Force Base during Young Hearts, as well as convince Janet to visit him at the Boomers camp.
 * Black Widow: You can convince Tyrone to give up the chemical ingredients in Don't Make a Beggar of Me.

Note: Depending on your faction of choice the effectiveness of these perks will vary. For example, if you took the Black Widow or Confirmed Bachelor perk you would encounter more female opponents in combat than male opponents when they belong to a mixed male/female faction such as the NCR. The same goes for the Cherchez La Femme and Lady Killer perks except you would encounter more male opponents than you would female opponents. None of the aforementioned in any way affects you if you side against Caesar's Legion as they are a male only faction.

Bugs

 * The Lady Killer speech option with Cass says "[Lady Killer] [Ladykiller]" instead of just "[Lady Killer]".

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