When a woman is equal to, almost the same height - or taller than her boyfriend, fiancé, or husband - she will very often discontinue wearing heels. This body language behavior is particularly strong in the context of a relatively new relationship. What are the reasons for this? - the woman wants to feel feminine, she also wants to make her man feel masculine - and she does not want to constantly remind him of his relative short stature. Instinctively she knows that doing so will lead to tensions of many sorts. Most women will say, "It just doesn't feel right!" to be taller than her partner.
Of course there are some women who never stop wearing heels when dating or when eventually they marry shorter men. They will not tolerate such social posturing and are thus very confident and alpha females (e.g. Carla Bruni) - but to make such a relationship last (a relative term to be sure when it comes to dating or marriage) they must be with a very confident man (e.g. Nicolas Sarkozy).
However when a woman who had initially adopted a "no heels" or short heels habit - converts back to wearing heels (with the exception of some professional settings), this is a warning sign that she no longer prioritizes their height disparity and thus indicates a drop of empathy towards her partner and signals an emotional rift, falling out of love or perhaps even infidelity (e.g., she wants to appear attractive to other potential or actual partners). Empathy is directly correlated with love and relationship longevity.
Moreover the Polish Anthropologist, Boguslaw Pawlowski, Ph.D., has noticed a very interesting predictor of both attraction as well as relationship longevity - Male to Female height ratio. If the man is 9% taller than the woman (1.09 times taller - or inversely stated - if the woman is 91.7% the height of the man) - this is the optimum height for attraction and relationship longevity.
It's interesting to note that Tom Cruise (5'7" [170.2 cm]) is 2 inches (3.08 cm) shorter than his then-wife Katie Holmes (5'9" [175.25 cm]). Their male-to-female ratio was only 0.97! Conversely, Paul Newman and Joanne Woodward - who were married for 50 years - had a ratio of exactly 1.09. Kevin Bacon and Kyra Sedgwick have a ratio of 1.085 and their marriage is going on 27 years.
Caroline Wozniacki is taller than her former fiancé - and is listed at 5'10" (177.8 cm) while Rory McIlroy is generously reported to be 5'9" (175.25 cm) - this would put their height ratio at 0.985. Yet looking at the above image, even when taking into account her heels and the fact he is closer to the camera - it seems she is taller than him by a greater margin of 1 inch - but more like 2 to 3 inches (putting this estimated ratio between 0.957 and 0.971). Of course these are only statistical correlations - and statistics do not govern the behavior of individual people, relationships, athletes, or atoms. Statistics govern large groups. So while there are many exceptions - yet these tenancies are very interesting and not to be ignored.
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Nonverbal Communication Analysis No. 2825: Rory McIlroy and Caroline Wozniacki break off their Engagement - Body Language Tells
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Nonverbal Communication Analysis No. 2701: Cuddling, Romance, Affection and Foot-Shunning
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Nonverbal Communication Analysis No. 2424: Giada De Laurentiis on Conan O'Brien Beautiful Teeth but Insincere Smiles and Some Body Language Signals of Waning Love