Validity in experimental studies can also be reduced by experimenter/researcher effects. This is when the researcher’s behaviour either intentionally or unintentionally alters the participants behaviour. For example, in the 44 thieves experiment, Bowlby (1944) has been criticised because he received information about the children before conducting his interviews. This means he may have altered the questions that he asked the mothers and the children as he had different expectations about them based on what he had been told by the social workers and psychologist. Likewise, Piaget has been criticised for asking children the same question twice in his famous conservation experiments, e.g. “Is there more or less or are they both the same”, “What about now?”

Children are often asked the same question twice in real life when they have made a mistake and the questioner wants them to reconsider their answer. So Piaget’s results may not indicate that the children really are not able to decentre but that they are responding to subtle contextual cues and attempting to please the researcher. They may believe that the situation demands a certain type of response. That is why these cues are referred to as demand characteristics. When a child responds to these cues this is called participant reactivity. Some participants may be more reactive than others but children are especially vulnerable to demand characteristics as they often wish to please adults as they are seen as authority figures. Susan Rose and Marion Blank (1074) replicated Piaget’s conservation tasks and compared Piaget’s standard task with a task in which children were only asked the question after the appearance of the objects had been changed. Performance was much improved suggesting that the children were able to decentre they simply gave Piaget answers they thought that he wanted to hear.