The merits of an application for custody must be determined on the basis of the best interests of the child. The best interests of the child are determined as per s. 24(2) of the Children’s Law Reform Act, by considering the child’s needs and circumstances, including:
(a) the love, affection and emotional ties between the child and:
(i) each person entitled to or claiming custody of or access to the child,
(ii) other members of the child’s family who reside with the child, and
(iii) persons involved in the child’s care and upbringing;
(b) the child’s views and preferences, if they can reasonably be ascertained;
(c) the length of time the child has lived in a stable home environment;
(d) the ability and willingness of each person applying for custody of the child to provide the child with guidance and education, the necessaries of life and any special needs of the child;
(e) the plan proposed by each person applying for custody of or access to the child for the child’s care and upbringing;
(f) the permanence and stability of the family unit with which it is proposed that the child will live;
(g) the ability of each person applying for custody of or access to the child to act as a parent; and
(h) the relationship by blood or through an adoption order between the child and each person who is a party to the application.
The courts will further consider under s. 24Â (3) and (4) the past conduct of the parents, especially considering a history of violence and abuse.
http://www.canlii.org/en/on/laws/stat/rso-1990-c-c12/latest/rso-1990-c-c12.html