Peel the potatoes, and cut them into chunks. Place the potatoes and the garlic in a 3-quart pot, add water just to cover, and bring to a boil over high heat. Reduce the heat to medium, cover, and cook for about 12 minutes, or until soft.
Drain the potatoes and garlic, reserving 1/2 cup of the cooking liquid. Return the vegetables to the pot, and stir in the yogurt or sour cream, salt, pepper, and 1/4 cup of the reserved cooking liquid.
Beat the potatoes with an electric mixer or mash with a potato masher until smooth. If the potatoes are too stiff, add enough of the reserved cooking liquid to achieve the desired consistency. Serve hot.