Ever noticed that people who buy one biscuit brand also buy several others? Marketers call this pattern the duplication of purchase law. It sounds heavy, but the idea is simple. Brands share their buyers with other brands, and the share depends on size. Big brands get a bigger slice of everyone’s basket. Small brands get a smaller slice. This one rule explains a lot about buyer behaviour and brand growth. In this guide, you will learn what the duplication of purchase law says, why it holds, and how Indian brands can use it. We will also look at real research and simple steps you can try this quarter.

1. What Is the Duplication of Purchase Law?

1.1 A Simple Definition

The duplication of purchase law says that brands share customers with rival brands in line with those rivals’ market share. Put simply, if a rival holds a bigger share, you will share more customers with it. If a rival is small, you will share fewer. Marketers shorten it to DoP. It works as a quick rule of thumb in any category where people buy more than one brand, like snacks, shampoo, or mobile plans. Importantly, the law does not say customers love every brand equally. It only says that overlap follows size. As a result, your biggest competitor is usually the biggest brand, not the one that looks most like you.

1.2 Where the Law Comes From

Andrew Ehrenberg and Gerald Goodhardt first spotted the pattern in the late 1960s, while studying TV viewing. Viewers who watched one channel also watched others, and the overlap tracked each channel’s audience size. Later studies found the same pattern in packaged goods, services, and business markets. The Ehrenberg-Bass Institute in Australia keeps testing the law across many categories. Professor Byron Sharp then brought these ideas to a wider audience through his book How Brands Grow. So far, the law has held up well. That long track record is why many marketers treat it as a law, not a passing trend.

1.3 A Simple Example With Made-Up Numbers

Let us use made-up numbers to see the law at work. Imagine a tea category in one city with three brands. Brand A is the leader, Brand B is mid-sized, and Brand C is small. Suppose 60% of tea buyers use A, 30% use B, and 10% use C. Now look at Brand C’s buyers. Roughly 60% of them will also buy A, and around 30% will also buy B. The same shape appears for Brand B’s buyers. Overlap with A is always the highest, because A has the most buyers. No brand is an island. Everyone shares shelves, and everyone shares customers.

2. How Buyer Behaviour Creates This Pattern

2.1 Most Buyers Use Many Brands

Real buyer behaviour is messy. Few people stay loyal to one brand for everything. You may use one toothpaste at home and another when you travel. Your family may buy two atta brands, depending on the offer. Most shoppers keep a small set of brands they find acceptable. Then they pick between them by habit, availability, and price. Because of this, almost every brand’s customers also buy other brands. That is normal, and it is not a sign of failure. Seeing buyer behaviour this way removes a big myth. Brands rarely own loyal customers. They mostly share customers with others.

2.2 Big Brands Have More Buyers, So They Share More

The logic is simple maths. A big brand has more buyers, so any group of customers is more likely to include some of them. Hence, every other brand shares more customers with it. A small brand has few buyers, so it shows up in fewer baskets. Think of a cricket crowd. A star player has fans from every team, while a new player has few fans anywhere. Brands work the same way. Size drives overlap, and overlap follows size in a steady, predictable curve. That predictability is exactly why researchers call it a law.

3. Key Ideas That Work Alongside the Law

3.1 Double Jeopardy

Another well-known pattern is the double jeopardy law. It says smaller brands suffer twice. They have far fewer buyers, and those buyers are also slightly less loyal. How Brands Grow shows UK washing powder data to prove it. Market leader Persil and smaller brand Surf had very different penetration, yet their average purchase frequency differed only a little. The same book compares Crest and Colgate toothpaste in the US, where Crest holds double Colgate’s share. Colgate’s lower numbers simply reflected its size, not a hidden problem. You can read asummary of the bookfor more detail.

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3.2 Penetration Over Loyalty

Research summarised in How Brands Grow shows that brands mostly grow by adding more buyers. They do not grow by making existing buyers buy far more. Loyalty programmes still have a place. However, they rarely change a brand’s position alone. Since customers already buy several brands, the easiest route is to reach more of them and win a bigger share of their purchases. That is why wide reach usually beats narrow targeting. In simple words, brand growth comes from being easy to notice and easy to buy for many people, not only for your fans.

3.3 Partitions: When Brands Share More Than Expected

Sometimes brands share more customers than the law predicts. Researchers call these groups partitions. A partition often forms when brands are truly alike, such as two aspirin brands. It can also form around a clear product type. Partitions are real, but they are usually small. Most categories still follow the main pattern. So before you chase a look-alike rival, check whether the data actually shows extra sharing. Otherwise, you may waste effort on a smaller threat while the market leaders quietly take your future buyers. Data beats gut feeling here.

4. What the Research Shows

4.1 Business Insurance in the US

Professor Jenni Romaniuk tested the law in US business insurance for LinkedIn’s B2B Institute. The result matched the pattern. On average,24% of customers at any company in that marketalso held a policy with State Farm, the brand with the highest penetration. Smaller insurers shared fewer customers with every rival. Romaniuk explained to Marketing Week that brands share more customers with bigger competitors and fewer with smaller ones. This study shows the duplication of purchase law works in B2B markets too, not only in shops and supermarkets. Size still decides who your real rivals are.

4.2 Proof From China

Researchers from the Ehrenberg-Bass Institute also tested the law in China. They studied six categories: two personal care, two impulse buys, and two durables. They used data from multiple years. According to theJournal of Product & Brand Management paper,the law held across all six categories. Brands shared customers in line with each rival’s market share, and few partitions showed extra sharing. This matters for India too. It suggests that the pattern comes from how people shop, not from one country’s habits. Even so, Indian brands should confirm it with their own data.

4.3 Pain Relief Tablets and Partitions

John Dawes studied the top eight US over-the-counter pain relief tablet brands. Overall, each brand shared its buyers with others in line with the other brand’s size. The study on SSRN also found partitions. Aspirin brands shared buyers with each other more than expected, and ibuprofen brands did so to a lesser extent. That finding shows how the law and partitions work together. The law explains the broad market, while partitions explain small pockets inside it. Together, they give marketers a clear map of who really competes with whom.

5. What the Law Means for Brand Growth

5.1 Compete With the Biggest Brands

If overlap follows size, your main rivals are the biggest brands in your category. Your future customers probably buy from them right now. So the question is not how to beat a similar small brand. The real question is how to win a bit more share from the leaders. Ehrenberg-Bass guidance on how B2B brands compete says growth comes from gaining customers from all other brands, in proportion to their share. The same thinking helps consumer brands. Brand growth starts when you look at where buyers truly are, not where your brand deck says they are.

5.2 Reach the Whole Market

Romaniuk’s work says that however niche your positioning, reaching the entire market is the most efficient way to grow. That may surprise brands that love tight targeting. Still, the data backs it. Buyers in your category already use many brands, so all of them are potential customers. Narrow targeting leaves many of them unreached. Instead, build wide reach with clear, consistent messages. Use simple cues that people recognise quickly. Over time, more buyers will think of you, and that lifts brand growth in a steady, lasting way.

5.3 Do Not Obsess Over Look-Alikes

Many brand teams spend months studying a rival that copies their look or price. Yet the data shows that a small look-alike is rarely the big threat. Ehrenberg-Bass advises brands not to get distracted by smaller look-alikes unless the numbers show extra sharing. Sharp’s profiling of hundreds of brands also found that a brand’s buyers typically differ from the category norm by only about two percentage points. In plain terms, almost every brand sells to the same kind of people. So spend your energy on reaching them, not on policing small rivals.

6. How to Use the Law in Your Own Category

6.1 Collect Overlap Data

Start with simple data. Use a buyer survey, panel data, or your own sales records. For each brand, ask which other brands its buyers also use. Then build a table that shows the overlap. In the table, the biggest brands should show the highest sharing with everyone. This is your baseline, the pattern you expect from duplication of purchase. Now compare each pair of brands against that baseline. Where overlap is much higher than expected, you have found something worth studying. Where overlap fits the pattern, you can relax and focus on reach.

6.2 Spot Partitions and Real Rivals

Next, look for partitions. They may point to a sub-market with special needs or tastes. For instance, buyers of herbal products might share more with other herbal brands. If so, those brands are closer rivals for that group. Plan special messages for that group, but keep your mass reach running. Also track the data over time, since overlap changes as brands grow or shrink. Finally, share a one-page summary with your team. Simple charts help non-research colleagues understand buyer behaviour quickly, and act on it.

7. How Influencers and UGC Help Brand Growth

7.1 Reach Beyond Your Own Fans

If wide reach is the goal, creators are a practical tool. Your own followers are usually people who already know you. Creators bring your brand to audiences you cannot reach alone. India’s influencer market keeps expanding. Kofluence’s 2026 report values the sector at ₹3,000-3,500 crore in 2025, with a 22% growth rate. It also says 93.1% of brands treat Instagram as their main influencer channel. For a brand that wants more buyers across the whole category, that scale is useful. It lets you reach shared buyers where they already spend time.

7.2 Many Creators, Many Audiences

Because buyers share many brands, a single message rarely wins. A mix of creators works better. Big names give broad reach. Mid-sized creators bring trust and relevance. Regional and niche creators speak local languages and match daily habits. UGC adds real faces using real products, which makes your brand look familiar and easy to pick. Together, these voices touch the same buyers who also shop your rivals. This is how brand growth can feel natural instead of forced. Finally, judge results by reach and new buyers, not only by likes.

8. Conclusion

8.1 Key Learnings

The duplication of purchase law explains how brands share customers: in line with size. Once you accept duplication of purchase as normal buyer behaviour, strategy gets simpler. Your main rivals are the biggest brands. Your best path to brand growth is wide reach, not narrow targeting. Research from business insurance, personal care, and pain relief shows the pattern holds across many markets. Use overlap data, watch for partitions, and keep your message consistent. Creators and UGC help you reach shared buyers at scale. Here are the main takeaways in short:

• Brands share customers in line with each rival’s market share.
• Your biggest rivals are usually the biggest brands, not the closest look-alikes.
• Smaller brands have fewer buyers, and those buyers are slightly less loyal.
• Growth mostly comes from adding more buyers, so aim for wide reach.
• Check overlap data for partitions before treating a rival as a special threat.
• Use a mix of influencers and UGC to reach buyers who also shop your competitors.

9. Frequently Asked Questions

9.1 What does duplication of purchase mean in simple words?

It means brands share their buyers with other brands, and the share depends on the other brand’s size. If a rival is big, you share many customers with it. If a rival is small, you share few. It shows that most buyers use several brands, not just one.

9.2 Who discovered the law?

Andrew Ehrenberg and Gerald Goodhardt first noticed the pattern in the late 1960s, while studying TV viewing. Later, the Ehrenberg-Bass Institute in Australia tested it across many categories and countries. Professor Byron Sharp made the wider idea popular through his book How Brands Grow.

9.3 What is a partition?

A partition is a small group of brands that share more customers with each other than the law predicts. Aspirin brands in the US are one example. Partitions usually form around similar products or clear product types. They are real, but most markets still follow the main pattern.

9.4 How is it different from brand loyalty?

Loyalty asks how often one buyer picks the same brand. This law asks which other brands those buyers also pick. Research shows most buyers are not fully loyal to one brand. They mix a few brands. So the law gives a more realistic picture of how people really shop.

9.5 Does it apply to B2B markets?

Yes. Professor Romaniuk’s study for LinkedIn’s B2B Institute found the same pattern in US business insurance. The Ehrenberg-Bass Institute also reports it in other business categories. B2B brands share customers with rivals in line with size, so reaching the whole market still matters.

9.6 Does it work in India?

Large public studies mostly come from markets like the US, the UK, and China. The China study found the law across six categories. Indian brands should test it with their own survey or panel data. The pattern comes from common shopping habits, so it is worth checking.

9.7 What is double jeopardy?

Double jeopardy says smaller brands lose twice. They have far fewer buyers, and those buyers are slightly less loyal. UK washing powder data showed this clearly. Persil and Surf differed hugely in penetration, but only a little in purchase frequency. It works hand in hand with the overlap pattern.

9.8 Why should small brands care?

Small brands often assume they can win by targeting a tiny niche. The law says their buyers also buy big brands, and new customers will mostly come from the big ones. So small brands should aim for broad reach, simple cues, and steady messages. That approach uses a limited budget more wisely.

9.9 How can I measure it?

Run a survey or use panel data. List the main brands in your category and ask which ones each buyer uses. Build a table of overlap between every pair of brands. Compare it with brand sizes. Pairs with much higher overlap than expected point to partitions or special rivalries.

9.10 Can influencer marketing help brands apply it?

Yes. Since buyers share many brands, you need broad reach with familiar faces. Influencers and UGC creators bring your brand to new audiences, in different languages and niches. Use a mix of big, mid-sized, and regional creators. Then track new buyers and reach, not only likes or views.

9. Ready to Reach More Buyers?

Brands do not grow by hiding in a niche. They grow by showing up wherever buyers already look. If you are a brand, register with Hobo.Video today and launch influencer and UGC campaigns that reach buyers across your whole category. If you are a creator, join our community of 2.25 million creators and start working with brands that match your voice. Your next campaign can start winning new buyers right now.

10. About Hobo.Video

Hobo.Videois India’s leading AI-powered influencer marketing and UGC company. With over 2.25 million creators, it delivers end-to-end campaign management built for brand growth. The platform blends AI with human strategy for strong ROI. Its services include:

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