  Machine ethics is the field that studies how ethical behaviour can be
accomplished by autonomous systems. While there exist some systematic reviews
aiming to consolidate the state of the art in machine ethics prior to 2020,
these tend to not include work that uses reinforcement learning agents as
entities whose ethical behaviour is to be achieved. The reason for this is that
only in the last years we have witnessed an increase in machine ethics studies
within reinforcement learning. We present here a systematic review of
reinforcement learning for machine ethics and machine ethics within
reinforcement learning. Additionally, we highlight trends in terms of ethics
specifications, components and frameworks of reinforcement learning, and
environments used to result in ethical behaviour. Our systematic review aims to
consolidate the work in machine ethics and reinforcement learning thus
completing the gap in the state of the art machine ethics landscape
