Psi is the phenomenon of apparently responding to or receiving information by means other than the recognised senses. Psi information may influence human behaviour, without the individual intending this or even being aware of it. This thesis seeks to investigate nonintentional behavioural responses to psi. We present five empirical studies that investigated nonintentional behavioural responses to psi information. In each study, the psi information was hidden from participants, in that the participants neither had sensory access to it, nor did they know that it existed. Two different combinations of psi information and a behavioural response were examined. The first was the influence of hidden psi information on psychological task performance. The second was the influence of covert, remote observation by hidden observers on the social facilitation effect. In all the studies, the effects of individual differences in participants’ personalities were also considered. In Experiment 1 we investigated whether hidden targets influenced participants’ judgements of the lengths of lines. There was no overall psi effect, but we found a replication of a response bias effect and a significant correlation between psi and participants’ extraversion. In Experiment 2 we investigated whether hidden targets influenced participants’ speed on a maths task. There was no overall psi effect and no correlations between personality and psi scores. We reviewed previous research literature on social facilitation from the novel angle of investigating whether being watched can, in and of itself, lead to the social facilitation effect. Experiments 3, 4, and 5 developed the paradigm of testing for a social facilitation effect from remote observation, investigating whether remote observation leads to the same behavioural changes as knowingly being observed by a physically present person. We compared participants’ performance on psychological tasks under different observation conditions: alone, remotely observed by a hidden observer, and observed by a physically present observer. The expected social facilitation effect was not found in these experiments, leading to a series of improvements to the sampling, methodology, and tasks over the course of these experiments. As the social facilitation effect from a physically present observer was not reliably replicated, these experiments were not conclusive tests of whether there is a social facilitation effect from remote observation. However, there was an indication in Experiment 3 that remote observation does not exert a significant behavioural effect. Considered together, our studies explored novel approaches to examining nonintentional behavioural responses to psi. The significant correlation between participants’ extraversion and psi is, to our knowledge, the first time this effect has been found in a nonintentional psi experiment. This, and the replication of the response bias effect, represent important advances in parapsychology. Our experiments are also the first to test the assumption, inherent in many research designs, that covert observation does not affect participants’ behaviour. Overall, our findings did not support the psi hypothesis.