Australian scientist Paul Hemsworth found that inconsistent handling stressed pigs and reduced weight gains. If the handler occasionally slapped or shocked the pigs when he entered the pens, the animals became stressed each time he approached. Even if the handler is gentle and nice most of the time, the pigs learned that he is not to be trusted. How an animal perceives a situation during handling can have a big effect on stress levels. Procedures such as weekly weighing will usually have little effect on weight gain if the animals have learned that nothing bad is going to happen.
Most animals are smart enough to associate certain people with painful experiences.
It is doubtful that baby piglets associate a particular person with castration. These animals are so young that the brain is probably too immature to store specific memory. Stimuli which may initially frighten a pig can often become pleasurable. During pen washing the animals squealed and became excited the first few times. By the third or forth washing the animals appeared to eagerly anticipate pen washing.