Exceptionally secretive, Ludwig only ever agreed to one interview, which he granted to Fortune magazine journalist Dero Saunders in 1957. Thereafter, the mysterious magnate who was as frugal as he was reclusive, kept his lips firmly shut and avoided the press like the plague.
By 1978, Ludwig was estimated to be worth around $12 billion ($9.9bn) in today's money, making him America's richest person, yet he remained almost unknown in the US. The invisible tycoon lived in a large apartment in Midtown Manhattan but even his closest neighbours had no idea who he was. Ludwig died at home at the age of 95 in 1992; Forbes estimated his wealth to be $1.2 billion at this time, the equivalent of $2.2 billion (£1.8bn) in today's money.
Often dubbed 'the archetypal super-rich hermit', American businessman Howard Hughes first made his money as a movie producer in the 1920s and 30s, working on films such as Scarface (1932). Hughes went on to make money as a property mogul, pioneering aviator and philanthropist, and in the process amassed enormous wealth, equivalent to $9 billion (£7.1bn) in today's money. This also garnered international celebrity status from the 1920s onwards, and he enjoyed an extravagant lifestyle and was something of a playboy in his younger days.
From there on in Hughes moved from hotel to hotel where he would watch the same movies over and over again and was seldom seen by anyone. By the time of his death in 1976, the genius entrepreneur was emaciated and unkempt with a grossly long beard and nails, the vision of an old hermit. Hughes' grave site in Houston is pictured here.
As outlined in the New York Times' bestseller Empty Mansions by NBC reporter Bill Dedman and Clark's cousin, Paul Clark Newell Jr, Clark's mental health began to deteriorate post-divorce and she spent most of her time with her mother, confined to the family's Fifth Avenue apartment or lavish country estates. Following her mother's death in 1963, Clark, who had developed intense paranoia, became even more reclusive.
The brothers went their separate ways business-wise in 1961 following a dispute over whether they should sell cigarettes, splitting the company into two distinct entities, Aldi Nord and Aldi Sud, but continued to maintain a personal relationship. Theo is shown here in 1971, but not long after the photo was taken, Theo was kidnapped and held for 17 days. After his release, the Aldi Nord boss and his brother became exceedingly security conscious with Forbes at one point describing the pair as being "more reclusive than a Yeti".
The siblings, who had a combined peak net worth of $45.7 billion (£37.7bn), remained acutely publicity-averse for the rest of their lives. Theo eventually died in 2010, while Karl passed away in 2014.
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Despite his extreme wealth, which peaked at $33 billion (£27.2bn) in 2007, the IKEA founder encouraged his staff to write on both sides of pieces of paper, berated them for leaving lights on and was all about cost-cutting in his private life too, choosing to drive a basic Volvo, only fly economy and shamelessly stock up on freebie salt and pepper sachets when he was eating out.
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After decamping to Switzerland in 1976 to avoid Sweden's punishing tax regime, the thrifty IKEA boss lived out the rest of his days in relative seclusion. He returned to Sweden in 2013 to spend more time with his family. He died five years later in 2018, with half of his wealth going to fund development projects in the north of his home country.
Camera-shy and reclusive, identical twin brothers Sir David (pictured right) and Sir Frederick Barclay controlled a formidable media empire in the UK, which includes the Telegraph Group and The Spectator magazine, and own other major interests from London's Ritz Hotel (now sold) to retailer Littlewoods and delivery firm Yodel. Sir David died this January at the age of 86. At the time of his death, Forbes estimated he and his children owned 75% of the family interests, which has led to a legal battle with Sir Frederick's children.
According to Forbes, Sir David had a net worth of $3.7 billion (£2.6bn) in 2020. The families live out of the public eye between the tax havens of Monaco and Brecqhou off the Channel Island of Sark, where they have built a mock-Gothic castle.
Ardent supporters of Brexit, the Barclays attracted criticism for their tax affairs, alleged editorial interference in the Telegraph Media Group and legal disputes with the Sark government. The private battle between the brothers' families has now become rather more public as details of alleged bugging of conversations were revealed in court.
Widely regarded as America's most reclusive billionaire, Philip Anschutz has spent the past five decades building up an enviable fortune, which spans oil, property, telecoms, and entertainment. Anschutz also has an interest in sport, and he owns both the National Hockey League's Los Angeles Kings team and a third of basketball's Lakers team, plus the building they play in, the Staples Center.
Currently worth a cool $10.1 billion (£7.7bn), Anschutz has made his money without fuss or fanfare and likes to keep himself to himself. According to Fortune magazine, the billionaire is painfully shy and loathes self-promotion. He has only ever granted three formal media interviews since 1979.
Australian mining heiress Angela Bennett is so spotlight-dodging and secretive, the press Down Under has nicknamed her "the night parrot". Bennett has only ever posed for one press photo (she is shown here on the left at a event organised by her son in December 2016), and has a habit of hiding her face from members of the paparazzi.
The mining scion has even gone as far as obtaining the copyright on images of an empty staircase at her former home in Perth, a home which she sold for a record $39 million back in 2009. Bennett seems to have taken after her late father Peter Wright, who was equally publicity-shy.
Bennett is now worth $1.4 billion (£990m), says Forbes, having fought a number of protracted legal battles over her fortune and lives a very quiet life indeed in the city of Perth (a neighbouring apartment to one owned by Bennett is shown here).
Mercer, whose net worth is estimated to be in the hundreds of millions at least, was the main investor in the data-mining firm Cambridge Analytica. This company reportedly played a controversial role in Donald Trump's election to government, and the 2016 Brexit campaign when it leant its services to Nigel Farage (pictured), the-then leader of the United Kingdom Independence Party (UKIP).
A supporter of right-wing causes in the US and elsewhere and an opponent of the Civil Rights Act of 1964, Mercer was one of the Trump campaign's biggest donors. However, the media scrutiny following these donations is said to be one of the main reasons behind his significantly smaller contributions to the Republicans in 2018. Mercer continued to support Trump in his 2020 election campaign, and in February 2020 donated his first six-figure amount – $355,200 (£270,950) – to the joint fundraising committee for Trump and the Republicans.
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