Rosamund Pike's Hair?

Actually, it is a hairstyle of the period, known as the "titus."

It wasn't a terribly conventional hairstyle though; it was worn by very modern, chic women.

...appreciate that information.
I remember in the movie Persuasian (1995), Lady Russell wears her hair in that style.

I wondered whether perhaps it was because of an illness and had to be cut for some reason? There is the scarlet fever scare at the end with the child so perhaps there was a deleted scene where Lady Harriet became ill and her hair was cut for whatever reason?

The change was just so sudden and strange that nothing makes sense , LOL!

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Again, a la Titus was a hairstyle from the early years of the nineteenth century; Jane Austen, who died in 1817, complained of the fashion associated with the French Revolution's Reign of Terror in some of her letters. By the time in which Wives and Daughters is set, a la Titus would have long passed from fashion. I looked at the first link you provided and could not find the reference, which would be completely out of place for the Victorian era, as well as for this discussion, since the novel takes place during the reign of William IV.

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I don't know much about the psychological trickery but I think Lady H's hairstyle is a knockout.
The story period is about 1831 i.e.King William 4ths reign. If you look closely at the scene where Molly and the squire are going through Osbourne's papers, Molly comes across the birth certificate of the squire's grandson.On the top rh corner there is a date of birth: 1829, the boy looks about 2, so I'm assuming the date is near enough correct.
Having said all that, the Titus hairstyle which others have suggested it is, is way out of date.

The short haircut, named "The Titus" came out in the early 1800's and refers to
the style meant to imitate the haircut of the unfortunates then about to be guillotined during the Terror in France 1793/1794.

You are correct re Lady H being the most attractive person in the film, and her nature is attractive as well.But Molly had a rather sweet mobile look about her.
And a lovely nature.
In fact the whole was a brilliant presentation.

Bah!Humbug! Imnottalez.

Sorry, crystal71061, but your explanation seems unsatisfactory. Why would the filmmakers, who tried to get the settings, the costumes and the majority of the hairstyles right, suddenly pull such a dim-witted trick? Someone of Lady Harriet's standing would not render herself hideous merely to make her friend look prettier. Justine Waddell is just as gorgeous as Rosamund Pike, so the whole argument does not hold water. The hairstyle is not even close to the period, since it had gone out of whatever tenuous fashion it ever enjoyed nearly twenty years earlier. I have a feeling some researcher made a mistake when looking up hairstyles. There is nothing in the novel to support the filmmakers' choice of an out-of-date hairstyle. People do not love each other merely for beauty, but also for shared interests, compatible personalities, etc. Roger and Molly fit the latter bill completely.

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