Schaffer’s research led to the formulation of four stages of developmental progress: Asocial (first few weeks) behaviour towards humans and inanimate objects is fairly similar, Indiscriminate attachments (2-7 months) preference for being with other humans but dont tend to have separation anxiety, Specificdiscriminate attachments (7-8 months) attachment towards one particular person and shows stranger anxiety, Multiple attachment (9 months +) extend attachments to people they often see.
His study shows good external validity, good inter-rater reliability, real world application and it was done on a large scale. However the mothers could’ve been bias, and it was only studied on 1960s working class Glasgow babies. There is also poor evidence for the asocial stage as babies are immobile so if they had anxiety it would be hard to observe.