Stereotypes and bias play a huge role in the short story, "Lamb to the Slaughter" by Roald Dahl. At the start of the story, the main character, Mary Maloney, is presented as the epitome of a stereotypical housewife- she waits patiently in the living room for her husband to come home from work, ready to serve him his drink and cook him dinner. She appears to have accepted this role. It is clear from the way that her husband speaks to her that he also sees her in this light, and treats her accordingly. He sees her as weak and obedient, and expects to be able to walk away from her with out any "fuss" as long as he gives her some money.
It is because he views her as this stereotype that he is caught unawares when she reacts violently to the news of his betrayal. He hollers at her to stop making his dinner, completely oblivious to the fact that she is walking up behind him wielding a weapon of frozen meat.
From that point, Mary uses this bias against her to her advantage. She knows that everyone in town, including her husband's fellow police officers, views her as a sweet and simple woman, and so she continues to play this part. She fixes her makeup, puts on a happy smile, and goes to the grocery store feigning a need for potatoes and peas for her beloved husband's dinner. She makes small talk with the grocer to establish an alibi. She then goes home and calls the police to say that she had just come home to find her husband dead.
Mary continues to play this part while the police are searching her house for evidence. When the first officer entered her home, "she fell right into Jack Noonan's arms, weeping hysterically." There is another bias at play here because she knows these men, and immediately reminds them of this by reacting in such a familiar way when they arrive. She continues to play the victim, weeping while feigning frailty and illness. One officer, completely convinced by her charade, asks, "if she wouldn't rather go somewhere else, to her sister's house perhaps, or to his own wife who would take care of her and put her up for the night." Instead of questioning her, they care for her, because it is the way that they have been trained to treat a delicate woman, especially a grieving widow. It is because of these biases and stereotypes that Mary is able to get away with her crime; it is impossible for these men to imagine this stereotypical housewife committing any sort of violent crime.