They speculated that:"if it were possible to contrast a minimal care situation with one involving highly individualized care, then one might get quite different results". In a paper published in 1961, "Maternal deprivation: Toward an empirical and conceptual re-evaluation", the paediatrician and psychologist Leon J. Yarrow conducted a definitive review of research and concluded that maternal separation had never been studied under pure conditions. Yarrow believed the complicating factors were always present. In Bowlby's book, "Attachment and Loss" there is a passing reference to the complexities of the institutional situation, there is a disappointing emphasis on the assertion that regardless of age and conditions of care, the young child's response to separation is usually the mourning sequence initiated by acute distress: "The subjects in the various studies differ, e.g. there is a variance in age, the type of home varies, the type of hospital or clinic they visit varies, the type of care they receive and the length of time they at the away from home. In spite of different backgrounds and expectations of the observers, the findings are remarkable in one aspect. That is once a child is aged six months, they tend to respond to the event of separation from mother in certain typical ways". Without citing the evidence regarding the influence of each class of variable, Bowlby asserts that "'by far the most important variable’" is the absence of the mother, and dismisses other variables as relatively unimportant. James and Joyce decided to try and determine the influence of variables on the behaviour of healthy young children during a ten-day separation from the mother. The couple decided to become foster parents to a series of young children by providing 24-hour support and make written and filmed observations of their reactions. James made a proposal to Bowlby, who at the time was director of the Tavistock clinic for new project that would look at separation in young children in much much more detail. In 1963, Bowlby assigned £1000 pounds for the new unit and in the same year, Robertson joined her husband at the Tavistock clinic as a research associate to work on a project that would be known as "Young Children in Brief Separation". The Robertson's started their work by conducting a comprehensive research review, similar to the type of review that Yarrow conducted. Their purpose in the project was to study the influence of variables including age, level of maturity and object constancy, previous parent-child relationships and quality of substitute care on responses of young children to separation from their mother, all to illustrate optimal substitute care.