There are 2 key research studies into the “sensitive period”. These were done by Konrad Lorenz (1935) and Klaus and Kennel (1976).
Arguably the most important variable to evaluate is the internal validity of a study. The 2 studies vary greatly in this. Konrad Lorenz had the hypothesis of “Newborn orphaned geese will imprint on the first large moving object they see”. His study involved separating a group of fertilised eggs into 2 groups. The experimental group was set up so that the first “large moving object” that they saw was Lorenz. The control group was also monitored to make sure that they first “large moving object” that they was their biological goose mother. This meant that the variable of “who the goslings saw first” was being manipulated. As this was completed with no confounding variables into the situation we come to the conclusion that the internal validity was high and the hypothesis he set out to test in the first place had been tested.
Klaus and Kennell set up an experiment involving observing a group of mothers and their children with the hypothesis that “increased skin to skin contact leads to closer attachments being formed”. They would do this by manipulating the variable of the amount of skin to skin contact given to the experimental group. The sample was split into a control group and an experimental group and the independent variable was influenced so that the control group received the usual amount of skin to skin contact and the experimental group, as displayed before, would receive 2-3 hours more skin to skin contact. They then observed the behaviours of the children, in their home environments, firstly within 2 weeks of the infants returning home and then in 8 months time. These behaviours were mainly based on “distress on separation” and “joy on reunion”. At first this may seem as though all extraneous variables have been controlled to the best of their ability however the variable of “additional hours of attention given in the hospital” confounded the results to such an extent that the internal validity was compromised completely and they ended up observing the results of this confounding variable. Both studies are field experiments that exhibit high ecological validity as I will mention later.
While on the subject of validity I think it is important to evaluate the external validity of these research studies into the “sensitive period”. It is important to notice the large links between external validity, generalisability and representability. In both studies we see that the external validity is low and this consequently brings problems generalising the study and issues with how representative the sample is of the general public.
In Lorenz’s study he sues baby geese, often known as goslings. This leads to questions on whether his findings about the sensitive period does in fact relate to humans. Obviously this means that the sample is not representative of the general public. This means that the external validity could be seen as low.
Klaus and Kennell’s study uses a sample that is often questioned amongst psychologists. The sample, consisting of young unmarried and socio-economically deprived mothers from a suburb of North America, did not represent the wider public of the time. This is said to cause the experiment to have low generalisability, low representability, and low external validity.
Next I would consider the ecological validity of the experiment. Few psychologists would consider debate the ecological validity of the studies as they were mostly done in an environment that is representative of the situation in which they would usually be found. However, questions were raised with both studies.
Although the control group for Lorenz’s study was left in the field environment, the experimental group was incubated in a lab to ensure that the first “large moving object” they saw when they hatched was Lorenz. Although this lab is not representative of the usual place that a goose would hatch it is seen as a situational variable that did not confound the results. This meant that Lorenz’s study had high ecological validity.
Klaus and Kennell’s study had very high ecological validity and in their study as their behaviour observations were done in the homes of the children and so was representative of the wider public’s environment in which a new mother would raise her child. However, the study is sometimes questioned about the fact that leaving the children in these uncontrolled environments for such a time allows many extraneous variables that could confound the results.
In conclusion we find that both studies had flaws and are often questioned on their validity. However we find that the concept of the “sensitive period” is then strengthened by another study by De Chateau, with a more representative sample of 42 Swedish middle class mothers and their infants, which found similar results to Lorenz and Klaus and Kennell. This means that both studies had high external reliability and this allows us to be confident in the concept of the “sensitive period”; even if the research studies left much to be desired.