Family units in any culture contain both ‘horizontal’ relationships between spouses and ‘vertical’ ones between parents and children. Many cultures had previously had a stronger emphasis on ‘vertical’ bonds, so that an emphasis on having offspring was often associated with arranged marriages and a younger age at marriage. By contrast, in Northwest Europe there was a greater focus on the ‘horizontal’ relationship, with marriage based on romantic love. More recently this pattern has spread around the world and is associated with cultural changes of other types too.