There are many things that can influence both the dependent and independent variables in this experiment. One example is if a child is malnourished. He would not form a secure attachment as a child and the malnourishment could lead to many bad outcomes later in life too. Similarly, if the child were to grow up in a good environment where he is well fed and taken care of, he would likely get attached to the mother and because of this good environment, would have a higher chance of having a "good outcome". This breaks the cause and effect relationship and says that a third variable causes both.
