MHC is a group of genes that play an important role in our immune system.  They are also linked to an individual's scent.  The genes are codominantly inherited - meaning that the more variation in the two mates' MHC, the more varied the immune system of the child.  This increases a child's chance of survival - and thus the ability to detect MHC in human relationships is advantageous for natural selection.

There were several variables that were controlled.  Women could not be on the pill because this changes their sense of smell.  In addition, males were not allowed to eat certain foods or use perfumes during this time so that their scents would not be altered.

It means that the women in the study did not know which MHC they were sniffing; in addition, the researchers did not know which woman had which t-shirt and which MHC she represented.  All this data was collected and coded and then the results were determined to avoid researcher bias.

Tests in animals - for example, rats - demonstrate that females are more attracted to male rats with a different MHC than their own.  In addition, it has been found that there is a higher rate of miscarriage in couples with similar MHC.