1. An image forming apparatus comprising: a charging section to which a charging voltage is applied, the charging section being configured to charge a photoconductor; a light exposure section configured to expose the photoconductor to light to form an electrostatic latent image; a development section to which a development voltage is applied, the development section being configured to develop the electrostatic latent image and form a toner image on the photoconductor; an image forming section configured to form a first test image on an image bearing member by a regular development scheme and a second test image on the image bearing member by a first bias development scheme, the regular development scheme being a scheme in which charging by the charging section to which a first charging voltage is applied, light exposure by the light exposure section and development by the development section to which a first development voltage is applied are performed to form a toner image, the first bias development scheme being a scheme in which charging by the charging section to which a second charging voltage lower than the first charging voltage is applied and development by the development section to which a second development voltage higher than the first development voltage is applied are performed but light exposure by the light exposure section is not performed to form a toner image; a concentration detection section configured to detect a concentration of the first test image and the second test image formed by the image forming section; and a causal part determination section configured to determine on a basis of a result of detection by the concentration detection section a causal part of an image defect that is caused when an image is output.