Pheromones

Pheromones are chemical substances released by one individual that can affect the behaviour or physiology of another member of the same species.

In animals, they are well established—for example, in mating and territorial marking—but the evidence for human pheromones is far less clear
In humans, they are thought to influence social and reproductive behaviors, including attraction

Major Histocompatibility Complex (MHC)

a set of genes involved in immune system function that influences odor (phermone) mate selection

Wedekind et al. (1995)

to investigate whether differences in the major histocombatibility complex (MHC) genes influece human mate preference

double-blind, quasi experiment
Used 49 female students and 44 male students (from different courses of the same university)
the participants were examined to establish their MHC types.
the women were asked to report if they consumed oral contraceptives because it may affect the perception of those women
the men were provided with a clean new 100% cotton t-shirt and were asked to wear it for two nights under controlled conditions (keeping it in a plastic bag when not wearing it, no perfume, spicy food, etc)
Female paritcipents then smelt the shirts and rated them on scale of 7 for intesnity and pleasentness
the shirts were:
MHC similar to female paritcipents
MHC dismilar to female participents
the testing was timed to the second week of the woman's menstrual cycle (most smell-sensitive)

Women generally rated the shirts with MHC dissimilar to them higher in pleasentness than the MHA similar to them
When women were on contreptives it was the oppsite

Body odor likely contains pheromone-like chemical signals reflecting genetic makeup.
These signals subconsciously influence perceived attractiveness.

The study shows that human behavior (attraction and mate selection) may be influenced by pheromones.
Specifically, individuals are biologically predisposed to prefer genetically dissimilar partners, increasing offspring immune strength.