"...it is generally accepted by those who are professionally concerned with children that, particularly in the early years, continuity of care is a most important part of a child's sense of security and that disruption to established bonds is to be avoided whenever it is possible to do so...". Ormrod LJ in D v M
So if one parent, either mother or father, is the primary carer the courts wouldn't change the status quo unless there is evidence that the child isn't surviving satisfactorily. The primary care is primarily determined by the existing working patterns and child care arrangements. If one parent works full time and the other doesn't work or works part time to fit around the children's needs the none/part time worker would be deemed the primary carer. If both parents work full time shared residence is indicated. Of secondary importance is who does most of the nuts and bolts of parenting - school runs, visits to the doctor and dentist, shopping for school uniform and shoes etc.