Olfactory System
<p>In women, the sense of olfaction is strongest around the time of <a href="http://en.wikipedia.org/wiki/Ovulation" title="Ovulation">ovulation</a>, significantly stronger than during other phases of the <a href="http://en.wikipedia.org/wiki/Menstrual_cycle" title="Menstrual cycle">menstrual cycle</a> and also stronger than the sense in males.<sup id="cite_ref-14" class="reference"><a href="http://en.wikipedia.org/wiki/Olfactory#cite_note-14">[15]</a></sup>
The <a href="http://en.wikipedia.org/wiki/Major_histocompatibility_complex" title="Major histocompatibility complex">MHC</a> genes (known as <a href="http://en.wikipedia.org/wiki/Human_leukocyte_antigen" title="Human leukocyte antigen">HLA</a> in humans) are a group of genes present in many animals and important for the <a href="http://en.wikipedia.org/wiki/Immune_system" title="Immune system">immune system</a>; in general, offspring from parents with differing MHC genes have a stronger immune system. Fish, mice and female humans are able to smell some aspect of the MHC genes of potential sex partners and prefer partners with MHC genes different from their own.<sup id="cite_ref-15" class="reference"><a href="http://en.wikipedia.org/wiki/Olfactory#cite_note-15">[16]</a></sup><sup id="cite_ref-16" class="reference"><a href="http://en.wikipedia.org/wiki/Olfactory#cite_note-16">[17]</a></sup></p>
<p>
Humans can detect individuals that are blood-related kin (mothers and children but not husbands and wives) from olfaction.<sup id="cite_ref-17" class="reference"><a href="http://en.wikipedia.org/wiki/Olfactory#cite_note-17">[18]</a></sup> Mothers can identify by body odor their biological children but not their stepchildren. Preadolescent children can olfactorily detect their full siblings but not half-siblings or step siblings and this might explain incest avoidance and the <a href="http://en.wikipedia.org/wiki/Westermarck_effect" title="Westermarck effect">Westermarck effect</a>.<sup id="cite_ref-18" class="reference"><a href="http://en.wikipedia.org/wiki/Olfactory#cite_note-18">[19]</a></sup> Functional imaging shows that this olfactory kinship detection process involves the frontal-temporal junction, the <a href="http://en.wikipedia.org/wiki/Insular_cortex" title="Insular cortex">insula</a>, and the dorsomedial <a href="http://en.wikipedia.org/wiki/Prefrontal_cortex" title="Prefrontal cortex">prefrontal cortex</a> but not the primary or secondary olfactory cortices, or the related <a href="http://en.wikipedia.org/wiki/Piriform_cortex" title="Piriform cortex">piriform cortex</a> or <a href="http://en.wikipedia.org/wiki/Orbitofrontal_cortex" title="Orbitofrontal cortex">orbitofrontal cortex</a>.<sup id="cite_ref-19" class="reference"><a href="http://en.wikipedia.org/wiki/Olfactory#cite_note-19">[20]</a></sup></p>